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158-645: The Big Bang is a physical theory that describes how the universe expanded from an initial state of high density and temperature . The notion of an expanding universe was first scientifically originated by physicist Alexander Friedmann in 1922 with the mathematical derivation of the Friedmann equations . The earliest empirical observation of the notion of an expanding universe is known as Hubble's Law , published in work by physicist Edwin Hubble in 1929, which discerned that galaxies are moving away from Earth at

316-413: A ). This can be understood intuitively: for an ordinary particle in a cube-shaped box, doubling the length of an edge of the box decreases the density (and hence energy density) by a factor of eight (2 ). For radiation, the decrease in energy density is greater, because an increase in spatial distance also causes a redshift. The final component is dark energy: it is an intrinsic property of space and has

474-439: A Belgian physicist and Roman Catholic priest , proposed that the recession of the nebulae was due to the expansion of the universe. He inferred the relation that Hubble would later observe, given the cosmological principle. In 1931, Lemaître went further and suggested that the evident expansion of the universe, if projected back in time, meant that the further in the past the smaller the universe was, until at some finite time in

632-528: A Russian cosmologist and mathematician , derived the Friedmann equations from the Einstein field equations, showing that the universe might be expanding in contrast to the static universe model advocated by Albert Einstein at that time. In 1924, American astronomer Edwin Hubble 's measurement of the great distance to the nearest spiral nebulae showed that these systems were indeed other galaxies. Starting that same year, Hubble painstakingly developed

790-483: A future horizon , which limits the events in the future that we will be able to influence. The presence of either type of horizon depends on the details of the Friedmann–Lemaître–Robertson–Walker (FLRW) metric that describes the expansion of the universe. Our understanding of the universe back to very early times suggests that there is a past horizon, though in practice our view is also limited by

948-435: A mathematical theory , in the sense that the word "theory" has a different meaning in mathematical terms. R i c = k g {\displaystyle \mathrm {Ric} =kg} The equations for an Einstein manifold , used in general relativity to describe the curvature of spacetime A physical theory involves one or more relationships between various measurable quantities. Archimedes realized that

1106-402: A working hypothesis . Dark energy In physical cosmology and astronomy , dark energy is a proposed form of energy that affects the universe on the largest scales. Its primary effect is to drive the accelerating expansion of the universe . Assuming that the lambda-CDM model of cosmology is correct, dark energy dominates the universe, contributing 68% of the total energy in

1264-404: A certain economy and elegance (compare to mathematical beauty ), a notion sometimes called " Occam's razor " after the 13th-century English philosopher William of Occam (or Ockham), in which the simpler of two theories that describe the same matter just as adequately is preferred (but conceptual simplicity may mean mathematical complexity). They are also more likely to be accepted if they connect

1422-434: A comprehensive explanation for a broad range of observed phenomena, including the abundance of light elements , the cosmic microwave background (CMB) radiation , and large-scale structure . The uniformity of the universe, known as the flatness problem , is explained through cosmic inflation : a sudden and very rapid expansion of space during the earliest moments. Extrapolating this cosmic expansion backward in time using

1580-616: A constant energy density, regardless of the dimensions of the volume under consideration ( ρ  ∝  a ). Thus, unlike ordinary matter, it is not diluted by the expansion of space. The evidence for dark energy is indirect but comes from three independent sources: In 1998, the High-Z Supernova Search Team published observations of Type Ia ("one-A") supernovae . In 1999, the Supernova Cosmology Project followed by suggesting that

1738-480: A direct estimate of the Hubble parameter The reliance on a differential quantity, ⁠ Δ z / Δ t ⁠ , brings more information and is appealing for computation: It can minimize many common issues and systematic effects. Analyses of supernovae and baryon acoustic oscillations (BAO) are based on integrals of the Hubble parameter, whereas ⁠ Δ z / Δ t ⁠ measures it directly. For these reasons, this method has been widely used to examine

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1896-500: A form of protoscience and others are a form of pseudoscience . The falsification of the original theory sometimes leads to reformulation of the theory. "Thought" experiments are situations created in one's mind, asking a question akin to "suppose you are in this situation, assuming such is true, what would follow?". They are usually created to investigate phenomena that are not readily experienced in every-day situations. Famous examples of such thought experiments are Schrödinger's cat ,

2054-766: A good example. For instance: " phenomenologists " might employ ( semi- ) empirical formulas and heuristics to agree with experimental results, often without deep physical understanding . "Modelers" (also called "model-builders") often appear much like phenomenologists, but try to model speculative theories that have certain desirable features (rather than on experimental data), or apply the techniques of mathematical modeling to physics problems. Some attempt to create approximate theories, called effective theories , because fully developed theories may be regarded as unsolvable or too complicated . Other theorists may try to unify , formalise, reinterpret or generalise extant theories, or create completely new ones altogether. Sometimes

2212-548: A more generic early hot, dense phase of the universe. In either case, "the Big Bang" as an event is also colloquially referred to as the "birth" of our universe since it represents the point in history where the universe can be verified to have entered into a regime where the laws of physics as we understand them (specifically general relativity and the Standard Model of particle physics ) work. Based on measurements of

2370-502: A mystery, and possible explanations abound. The main candidates are a cosmological constant (representing a constant energy density filling space homogeneously) and scalar fields (dynamic quantities having energy densities that vary in time and space) such as quintessence or moduli . A cosmological constant would remain constant across time and space, while scalar fields can vary. Yet other possibilities are interacting dark energy, an observational effect, and cosmological coupling (see

2528-498: A new branch of mathematics: infinite, orthogonal series . Modern theoretical physics attempts to unify theories and explain phenomena in further attempts to understand the Universe , from the cosmological to the elementary particle scale. Where experimentation cannot be done, theoretical physics still tries to advance through the use of mathematical models. Mainstream theories (sometimes referred to as central theories ) are

2686-503: A non-standard form of kinetic energy such as a negative kinetic energy . They can have unusual properties: phantom energy , for example, can cause a Big Rip . A group of researchers argued in 2021 that observations of the Hubble tension may imply that only quintessence models with a nonzero coupling constant are viable. This class of theories attempts to come up with an all-encompassing theory of both dark matter and dark energy as

2844-451: A process in the very early universe has reached thermal equilibrium is the ratio between the rate of the process (usually rate of collisions between particles) and the Hubble parameter . The larger the ratio, the more time particles had to thermalize before they were too far away from each other. According to the Big Bang models, the universe at the beginning was very hot and very compact, and since then it has been expanding and cooling. In

3002-508: A profound effect on the universe, making up 68% of universal density in spite of being so dilute, is that it is believed to uniformly fill otherwise empty space. The vacuum energy , that is, the particle-antiparticle pairs generated and mutually annihilated within a time frame in accord with Heisenberg's uncertainty principle in the energy-time formulation, has been often invoked as the main contribution to dark energy. The mass–energy equivalence postulated by general relativity implies that

3160-468: A rate that accelerates proportionally with distance. Independent of Friedmann's work, and independent of Hubble's observations, physicist Georges Lemaître proposed that the universe emerged from a "primeval atom " in 1931, introducing the modern notion of the Big Bang. Various cosmological models of the Big Bang explain the evolution of the observable universe from the earliest known periods through its subsequent large-scale form. These models offer

3318-465: A series of distance indicators, the forerunner of the cosmic distance ladder , using the 100-inch (2.5 m) Hooker telescope at Mount Wilson Observatory . This allowed him to estimate distances to galaxies whose redshifts had already been measured, mostly by Slipher. In 1929, Hubble discovered a correlation between distance and recessional velocity —now known as Hubble's law. Independently deriving Friedmann's equations in 1927, Georges Lemaître ,

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3476-498: A ship floats by displacing its mass of water, Pythagoras understood the relation between the length of a vibrating string and the musical tone it produces. Other examples include entropy as a measure of the uncertainty regarding the positions and motions of unseen particles and the quantum mechanical idea that ( action and) energy are not continuously variable. Theoretical physics consists of several different approaches. In this regard, theoretical particle physics forms

3634-430: A single phenomenon that modifies the laws of gravity at various scales. This could, for example, treat dark energy and dark matter as different facets of the same unknown substance, or postulate that cold dark matter decays into dark energy. Another class of theories that unifies dark matter and dark energy are suggested to be covariant theories of modified gravities. These theories alter the dynamics of spacetime such that

3792-447: A surrounding space, the Big Bang only describes the intrinsic expansion of the contents of the universe. Another issue pointed out by Santhosh Mathew is that bang implies sound, which is not an important feature of the model. An attempt to find a more suitable alternative was not successful. The Big Bang models developed from observations of the structure of the universe and from theoretical considerations. In 1912, Vesto Slipher measured

3950-424: A temperature of approximately 10 degrees Celsius. Even the very concept of a particle breaks down in these conditions. A proper understanding of this period awaits the development of a theory of quantum gravity . The Planck epoch was succeeded by the grand unification epoch beginning at 10 seconds, where gravitation separated from the other forces as the universe's temperature fell. At approximately 10 seconds into

4108-435: A universe which contracts slightly will continue contracting. According to Einstein, "empty space" can possess its own energy. Because this energy is a property of space itself, it would not be diluted as space expands. As more space comes into existence, more of this energy-of-space would appear, thereby causing accelerated expansion. These sorts of disturbances are inevitable, due to the uneven distribution of matter throughout

4266-469: A wide range of phenomena. Testing the consequences of a theory is part of the scientific method . Physical theories can be grouped into three categories: mainstream theories , proposed theories and fringe theories . Theoretical physics began at least 2,300 years ago, under the Pre-socratic philosophy , and continued by Plato and Aristotle , whose views held sway for a millennium. During

4424-493: Is accelerating , an observation attributed to an unexplained phenomenon known as dark energy . The Big Bang models offer a comprehensive explanation for a broad range of observed phenomena, including the abundances of the light elements , the CMB , large-scale structure , and Hubble's law . The models depend on two major assumptions: the universality of physical laws and the cosmological principle . The universality of physical laws

4582-419: Is assumed to be cold. (Warm dark matter is ruled out by early reionization .) This CDM is estimated to make up about 23% of the matter/energy of the universe, while baryonic matter makes up about 4.6%. In an "extended model" which includes hot dark matter in the form of neutrinos, then the "physical baryon density" Ω b h 2 {\displaystyle \Omega _{\text{b}}h^{2}}

4740-401: Is called "cosmological coupling" because the black holes couple to a cosmological requirement. Other astrophysicists are skeptical, with a variety of papers claiming that the theory fails to explain other observations. The evidence for dark energy is heavily dependent on the theory of general relativity. Therefore, it is conceivable that a modification to general relativity also eliminates

4898-414: Is close to flat . For the shape of the universe to be flat, the mass–energy density of the universe must be equal to the critical density . The total amount of matter in the universe (including baryons and dark matter ), as measured from the cosmic microwave background spectrum, accounts for only about 30% of the critical density. This implies the existence of an additional form of energy to account for

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5056-581: Is considered the age of the universe . There remain aspects of the observed universe that are not yet adequately explained by the Big Bang models. After its initial expansion, the universe cooled sufficiently to allow the formation of subatomic particles , and later atoms . The unequal abundances of matter and antimatter that allowed this to occur is an unexplained effect known as baryon asymmetry . These primordial elements—mostly hydrogen , with some helium and lithium —later coalesced through gravity , forming early stars and galaxies. Astronomers observe

5214-403: Is estimated at 0.023. (This is different from the 'baryon density' Ω b {\displaystyle \Omega _{\text{b}}} expressed as a fraction of the total matter/energy density, which is about 0.046.) The corresponding cold dark matter density Ω c h 2 {\displaystyle \Omega _{\text{c}}h^{2}} is about 0.11, and

5372-424: Is interpreted as a Doppler shift, the recessional velocity of the object can be calculated. For some galaxies, it is possible to estimate distances via the cosmic distance ladder . When the recessional velocities are plotted against these distances, a linear relationship known as Hubble's law is observed: v = H 0 D {\displaystyle v=H_{0}D} where Hubble's law implies that

5530-463: Is judged by the extent to which its predictions agree with empirical observations. The quality of a physical theory is also judged on its ability to make new predictions which can be verified by new observations. A physical theory differs from a mathematical theorem in that while both are based on some form of axioms , judgment of mathematical applicability is not based on agreement with any experimental results. A physical theory similarly differs from

5688-478: Is known. This allows the object's distance to be measured from its actual observed brightness, or apparent magnitude . Type Ia supernovae are the best-known standard candles across cosmological distances because of their extreme and consistent luminosity . Recent observations of supernovae are consistent with a universe made up 71.3% of dark energy and 27.4% of a combination of dark matter and baryonic matter . The theory of large-scale structure , which governs

5846-409: Is measured as a function of cosmological redshift . OHD directly tracks the expansion history of the universe by taking passively evolving early-type galaxies as "cosmic chronometers". From this point, this approach provides standard clocks in the universe. The core of this idea is the measurement of the differential age evolution as a function of redshift of these cosmic chronometers. Thus, it provides

6004-434: Is minimally coupled to gravity, and does not feature higher order operations in its Lagrangian. No evidence of quintessence is yet available, nor has it been ruled out. It generally predicts a slightly slower acceleration of the expansion of the universe than the cosmological constant. Some scientists think that the best evidence for quintessence would come from violations of Einstein's equivalence principle and variation of

6162-547: Is modeled by a cosmological constant term in Einstein field equations of general relativity, but its composition and mechanism are unknown. More generally, the details of its equation of state and relationship with the Standard Model of particle physics continue to be investigated both through observation and theory. All of this cosmic evolution after the inflationary epoch can be rigorously described and modeled by

6320-417: Is more hypothetical than that of dark matter, and many things about it remain in the realm of speculation. Dark energy is thought to be very homogeneous and not dense , and is not known to interact through any of the fundamental forces other than gravity . Since it is rarefied and un-massive—roughly 10  kg/m —it is unlikely to be detectable in laboratory experiments. The reason dark energy can have such

6478-474: Is no preferred (or special) observer or vantage point. To this end, the cosmological principle has been confirmed to a level of 10 via observations of the temperature of the CMB. At the scale of the CMB horizon, the universe has been measured to be homogeneous with an upper bound on the order of 10% inhomogeneity, as of 1995. An important feature of the Big Bang spacetime is the presence of particle horizons . Since

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6636-409: Is one of the underlying principles of the theory of relativity . The cosmological principle states that on large scales the universe is homogeneous and isotropic —appearing the same in all directions regardless of location. These ideas were initially taken as postulates, but later efforts were made to test each of them. For example, the first assumption has been tested by observations showing that

6794-500: Is possible to investigate the effect of dark energy in the history of the universe, and constrain parameters of the equation of state of dark energy. To that end, several models have been proposed. One of the most popular models is the Chevallier–Polarski–Linder model (CPL). Some other common models are Barboza & Alcaniz (2008), Jassal et al. (2005), Wetterich. (2004), and Oztas et al. (2018). Researchers using

6952-400: Is sometimes labeled "gravitational repulsion". In standard cosmology, there are three components of the universe: matter, radiation, and dark energy. This matter is anything whose energy density scales with the inverse cube of the scale factor, i.e., ρ  ∝  a , while radiation is anything whose energy density scales to the inverse fourth power of the scale factor ( ρ  ∝ 

7110-462: Is that it is an intrinsic, fundamental energy of space. This is the cosmological constant, usually represented by the Greek letter Λ (Lambda, hence the name Lambda-CDM model ). Since energy and mass are related according to the equation E = mc , Einstein's theory of general relativity predicts that this energy will have a gravitational effect. It is sometimes called vacuum energy because it

7268-448: Is the stress–energy tensor , which contains both the energy (or matter) density of a substance and its pressure. In the Friedmann–Lemaître–Robertson–Walker metric , it can be shown that a strong constant negative pressure ( i.e., tension) in all the universe causes an acceleration in the expansion if the universe is already expanding, or a deceleration in contraction if the universe is already contracting. This accelerating expansion effect

7426-415: Is the energy density of empty space – of vacuum . A major outstanding problem is that the same quantum field theories predict a huge cosmological constant , about 120  orders of magnitude too large. This would need to be almost, but not exactly, cancelled by an equally large term of the opposite sign. Some supersymmetric theories require a cosmological constant that is exactly zero. Also, it

7584-432: Is the proper distance, v {\displaystyle v} is the recessional velocity, and v {\displaystyle v} , H {\displaystyle H} , and D {\displaystyle D} vary as the universe expands (hence we write H 0 {\displaystyle H_{0}} to denote the present-day Hubble "constant"). For distances much smaller than

7742-414: Is thought to have completely ended when the universe was just a fraction of a second old. It is unclear what relation, if any, exists between dark energy and inflation. Even after inflationary models became accepted, the cosmological constant was thought to be irrelevant to the current universe. Nearly all inflation models predict that the total (matter+energy) density of the universe should be very close to

7900-399: Is unknown whether there is a metastable vacuum state in string theory with a positive cosmological constant, and it has been conjectured by Ulf Danielsson et al. that no such state exists. This conjecture would not rule out other models of dark energy, such as quintessence, that could be compatible with string theory. In quintessence models of dark energy, the observed acceleration of

8058-580: The Australian Astronomical Observatory scanned the galaxies to determine their redshift. Then, by exploiting the fact that baryon acoustic oscillations have left voids regularly of ≈150 Mpc diameter, surrounded by the galaxies, the voids were used as standard rulers to estimate distances to galaxies as far as 2,000 Mpc (redshift 0.6), allowing for accurate estimate of the speeds of galaxies from their redshift and distance. The data confirmed cosmic acceleration up to half of

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8216-647: The Dark Energy Spectroscopic Instrument (DESI) to make the largest 3-D map of the universe as of 2024, have obtained an expansion history that has greater than 1% precision. From this level of detail, DESI Director Michael Levi stated: We're also seeing some potentially interesting differences that could indicate that dark energy is evolving over time. Those may or may not go away with more data, so we're excited to start analyzing our three-year dataset soon. Some alternatives to dark energy, such as inhomogeneous cosmology , aim to explain

8374-539: The EPR thought experiment , simple illustrations of time dilation , and so on. These usually lead to real experiments designed to verify that the conclusion (and therefore the assumptions) of the thought experiments are correct. The EPR thought experiment led to the Bell inequalities , which were then tested to various degrees of rigor , leading to the acceptance of the current formulation of quantum mechanics and probabilism as

8532-534: The Friedmann-Robertson-Walker metric (which describes the isotropic and homogeneous universe that is the basic assumption of modern cosmology), then one finds that black holes gain mass as the universe expands. The rate is measured to be ∝ a , where a is the scale factor . This particular rate means that the energy density of black holes remains constant over time, mimicking dark energy (see Dark_energy#Technical_definition ). The theory

8690-529: The Hubble Space Telescope and WMAP. Cosmologists now have fairly precise and accurate measurements of many of the parameters of the Big Bang model, and have made the unexpected discovery that the expansion of the universe appears to be accelerating. "[The] big bang picture is too firmly grounded in data from every area to be proved invalid in its general features." — Lawrence Krauss The earliest and most direct observational evidence of

8848-635: The Lambda-CDM model . Some people argue that the only indications for the existence of dark energy are observations of distance measurements and their associated redshifts. Cosmic microwave background anisotropies and baryon acoustic oscillations serve only to demonstrate that distances to a given redshift are larger than would be expected from a "dusty" Friedmann–Lemaître universe and the local measured Hubble constant. Supernovae are useful for cosmology because they are excellent standard candles across cosmological distances. They allow researchers to measure

9006-585: The Lorentz transformation which left Maxwell's equations invariant, but was apparently uninterested in the Michelson–Morley experiment on Earth 's drift through a luminiferous aether . Conversely, Einstein was awarded the Nobel Prize for explaining the photoelectric effect , previously an experimental result lacking a theoretical formulation. A physical theory is a model of physical events. It

9164-487: The Milne model , the oscillatory universe (originally suggested by Friedmann, but advocated by Albert Einstein and Richard C. Tolman ) and Fritz Zwicky 's tired light hypothesis. After World War II , two distinct possibilities emerged. One was Fred Hoyle's steady-state model, whereby new matter would be created as the universe seemed to expand. In this model the universe is roughly the same at any point in time. The other

9322-715: The Planck spacecraft and the Supernova Legacy Survey. First results from the SNLS reveal that the average behavior (i.e., equation of state) of dark energy behaves like Einstein's cosmological constant to a precision of 10%. Recent results from the Hubble Space Telescope Higher-Z Team indicate that dark energy has been present for at least 9 billion years and during the period preceding cosmic acceleration. The nature of dark energy

9480-572: The Scientific Revolution gathered pace, the concepts of matter , energy, space, time and causality slowly began to acquire the form we know today, and other sciences spun off from the rubric of natural philosophy . Thus began the modern era of theory with the Copernican paradigm shift in astronomy, soon followed by Johannes Kepler 's expressions for planetary orbits, which summarized the meticulous observations of Tycho Brahe ;

9638-436: The critical density . During the 1980s, most cosmological research focused on models with critical density in matter only, usually 95% cold dark matter (CDM) and 5% ordinary matter (baryons). These models were found to be successful at forming realistic galaxies and clusters, but some problems appeared in the late 1980s: in particular, the model required a value for the Hubble constant lower than preferred by observations, and

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9796-486: The dwarf galaxy problem of cold dark matter. Dark energy is also an area of intense interest for scientists, but it is not clear whether direct detection of dark energy will be possible. Inflation and baryogenesis remain more speculative features of current Big Bang models. Viable, quantitative explanations for such phenomena are still being sought. These are unsolved problems in physics. Observations of distant galaxies and quasars show that these objects are redshifted:

9954-471: The equation of state had possibly crossed the cosmological constant boundary (w = −1) from above to below. A no-go theorem has been proved that this scenario requires models with at least two types of quintessence. This scenario is the so-called Quintom scenario . Some special cases of quintessence are phantom energy , in which the energy density of quintessence actually increases with time, and k-essence (short for kinetic quintessence) which has

10112-400: The expansion of the universe are required to understand how the expansion rate changes over time and space. In general relativity, the evolution of the expansion rate is estimated from the curvature of the universe and the cosmological equation of state (the relationship between temperature, pressure, and combined matter, energy, and vacuum energy density for any region of space). Measuring

10270-461: The two-fluid theory of electricity are two cases in this point. However, an exception to all the above is the wave–particle duality , a theory combining aspects of different, opposing models via the Bohr complementarity principle . Physical theories become accepted if they are able to make correct predictions and no (or few) incorrect ones. The theory should have, at least as a secondary objective,

10428-416: The universe's expansion is accelerating . Prior to this observation, scientists thought that the gravitational attraction of matter and energy in the universe would cause the universe's expansion to slow over time. Since the discovery of accelerating expansion, several independent lines of evidence have been discovered that support the existence of dark energy. The exact nature of dark energy remains

10586-407: The "four pillars" of the Big Bang models. Precise modern models of the Big Bang appeal to various exotic physical phenomena that have not been observed in terrestrial laboratory experiments or incorporated into the Standard Model of particle physics. Of these features, dark matter is currently the subject of most active laboratory investigations. Remaining issues include the cuspy halo problem and

10744-457: The 1978 Nobel Prize in Physics . Physical theory The advancement of science generally depends on the interplay between experimental studies and theory . In some cases, theoretical physics adheres to standards of mathematical rigour while giving little weight to experiments and observations. For example, while developing special relativity , Albert Einstein was concerned with

10902-429: The 19th century. Another important event in the 19th century was the discovery of electromagnetic theory , unifying the previously separate phenomena of electricity, magnetism and light. The pillars of modern physics , and perhaps the most revolutionary theories in the history of physics, have been relativity theory and quantum mechanics . Newtonian mechanics was subsumed under special relativity and Newton's gravity

11060-461: The Big Bang. Since the early universe did not immediately collapse into a multitude of black holes, matter at that time must have been very evenly distributed with a negligible density gradient . The earliest phases of the Big Bang are subject to much speculation, given the lack of available data. In the most common models the universe was filled homogeneously and isotropically with a very high energy density and huge temperatures and pressures , and

11218-530: The Big Bang. Then, from the 1970s to the 1990s, cosmologists worked on characterizing the features of the Big Bang universe and resolving outstanding problems. In 1981, Alan Guth made a breakthrough in theoretical work on resolving certain outstanding theoretical problems in the Big Bang models with the introduction of an epoch of rapid expansion in the early universe he called "inflation". Meanwhile, during these decades, two questions in observational cosmology that generated much discussion and disagreement were over

11376-476: The Dutchmen Snell and Huygens. In the 18th and 19th centuries Joseph-Louis Lagrange , Leonhard Euler and William Rowan Hamilton would extend the theory of classical mechanics considerably. They picked up the interactive intertwining of mathematics and physics begun two millennia earlier by Pythagoras. Among the great conceptual achievements of the 19th and 20th centuries were the consolidation of

11534-587: The Earth ) or may be an alternative model that provides answers that are more accurate or that can be more widely applied. In the latter case, a correspondence principle will be required to recover the previously known result . Sometimes though, advances may proceed along different paths. For example, an essentially correct theory may need some conceptual or factual revisions; atomic theory , first postulated millennia ago (by several thinkers in Greece and India ) and

11692-540: The Lambda-CDM model then became the leading model. Soon after, dark energy was supported by independent observations: in 2000, the BOOMERanG and Maxima cosmic microwave background experiments observed the first acoustic peak in the cosmic microwave background, showing that the total (matter+energy) density is close to 100% of critical density. Then in 2001, the 2dF Galaxy Redshift Survey gave strong evidence that

11850-473: The Universe began when it did. If acceleration began earlier in the universe, structures such as galaxies would never have had time to form, and life, at least as we know it, would never have had a chance to exist. Proponents of the anthropic principle view this as support for their arguments. However, many models of quintessence have a so-called "tracker" behavior, which solves this problem. In these models,

12008-463: The absence of a perfect cosmological principle , extrapolation of the expansion of the universe backwards in time using general relativity yields an infinite density and temperature at a finite time in the past. This irregular behavior, known as the gravitational singularity , indicates that general relativity is not an adequate description of the laws of physics in this regime. Models based on general relativity alone cannot fully extrapolate toward

12166-464: The accelerated cosmic expansion and study properties of dark energy. Dark energy's status as a hypothetical force with unknown properties makes it an active target of research. The problem is attacked from a variety of angles, such as modifying the prevailing theory of gravity (general relativity), attempting to pin down the properties of dark energy, and finding alternative ways to explain the observational data. The simplest explanation for dark energy

12324-507: The age measured today). This issue was later resolved when new computer simulations, which included the effects of mass loss due to stellar winds , indicated a much younger age for globular clusters. Significant progress in Big Bang cosmology has been made since the late 1990s as a result of advances in telescope technology as well as the analysis of data from satellites such as the Cosmic Background Explorer (COBE),

12482-411: The age of the universe (7 billion years) and constrain its inhomogeneity to 1 part in 10. This provides a confirmation to cosmic acceleration independent of supernovae. The existence of dark energy, in whatever form, is needed to reconcile the measured geometry of space with the total amount of matter in the universe. Measurements of cosmic microwave background anisotropies indicate that the universe

12640-419: The applications of relativity to problems in astronomy and cosmology respectively . All of these achievements depended on the theoretical physics as a moving force both to suggest experiments and to consolidate results — often by ingenious application of existing mathematics, or, as in the case of Descartes and Newton (with Leibniz ), by inventing new mathematics. Fourier's studies of heat conduction led to

12798-411: The big-bang predictions by Alpher, Herman and Gamow around 1950. Through the 1970s, the radiation was found to be approximately consistent with a blackbody spectrum in all directions; this spectrum has been redshifted by the expansion of the universe, and today corresponds to approximately 2.725 K. This tipped the balance of evidence in favor of the Big Bang model, and Penzias and Wilson were awarded

12956-514: The body of knowledge of both factual and scientific views and possess a usual scientific quality of the tests of repeatability, consistency with existing well-established science and experimentation. There do exist mainstream theories that are generally accepted theories based solely upon their effects explaining a wide variety of data, although the detection, explanation, and possible composition are subjects of debate. The proposed theories of physics are usually relatively new theories which deal with

13114-418: The competing theories successfully explain observations to the same level of precision as standard dark energy. Cosmologists estimate that the acceleration began roughly 5 billion years ago. Before that, it is thought that the expansion was decelerating, due to the attractive influence of matter. The density of dark matter in an expanding universe decreases more quickly than dark energy, and eventually

13272-431: The corresponding neutrino density Ω v h 2 {\displaystyle \Omega _{\text{v}}h^{2}} is estimated to be less than 0.0062. Independent lines of evidence from Type Ia supernovae and the CMB imply that the universe today is dominated by a mysterious form of energy known as dark energy , which appears to homogeneously permeate all of space. Observations suggest that 73% of

13430-508: The cosmic microwave background aligned with vast supervoids and superclusters. This so-called late-time Integrated Sachs–Wolfe effect (ISW) is a direct signal of dark energy in a flat universe. It was reported at high significance in 2008 by Ho et al. and Giannantonio et al. A new approach to test evidence of dark energy through observational Hubble constant data (OHD), also known as cosmic chronometers, has gained significant attention in recent years. The Hubble constant, H ( z ),

13588-437: The dark energy dominates. Specifically, when the volume of the universe doubles, the density of dark matter is halved, but the density of dark energy is nearly unchanged (it is exactly constant in the case of a cosmological constant). Projections into the future can differ radically for different models of dark energy. For a cosmological constant, or any other model that predicts that the acceleration will continue indefinitely,

13746-450: The determination of the Hubble constant is known as Hubble tension . Techniques based on observation of the CMB suggest a lower value of this constant compared to the quantity derived from measurements based on the cosmic distance ladder. In 1964, Arno Penzias and Robert Wilson serendipitously discovered the cosmic background radiation, an omnidirectional signal in the microwave band. Their discovery provided substantial confirmation of

13904-446: The equation of state for dark energy is one of the biggest efforts in observational cosmology today. Adding the cosmological constant to cosmology's standard FLRW metric leads to the Lambda-CDM model, which has been referred to as the " standard model of cosmology " because of its precise agreement with observations. As of 2013, the Lambda-CDM model is consistent with a series of increasingly rigorous cosmological observations, including

14062-444: The expansion history of the universe by looking at the relationship between the distance to an object and its redshift , which gives how fast it is receding from us. The relationship is roughly linear, according to Hubble's law . It is relatively easy to measure redshift, but finding the distance to an object is more difficult. Usually, astronomers use standard candles: objects for which the intrinsic brightness, or absolute magnitude ,

14220-483: The expansion of the universe is accelerating . The 2011 Nobel Prize in Physics was awarded to Saul Perlmutter , Brian P. Schmidt , and Adam G. Riess for their leadership in the discovery. Since then, these observations have been corroborated by several independent sources. Measurements of the cosmic microwave background , gravitational lensing , and the large-scale structure of the cosmos , as well as improved measurements of supernovae, have been consistent with

14378-590: The expansion using Type Ia supernovae and measurements of temperature fluctuations in the cosmic microwave background, the time that has passed since that event—known as the " age of the universe "—is 13.8 billion years. Despite being extremely dense at this time—far denser than is usually required to form a black hole —the universe did not re-collapse into a singularity. Commonly used calculations and limits for explaining gravitational collapse are usually based upon objects of relatively constant size, such as stars, and do not apply to rapidly expanding space such as

14536-400: The expansion, a phase transition caused a cosmic inflation , during which the universe grew exponentially , unconstrained by the light speed invariance , and temperatures dropped by a factor of 100,000. This concept is motivated by the flatness problem , where the density of matter and energy is very close to the critical density needed to produce a flat universe . That is, the shape of

14694-413: The first Doppler shift of a " spiral nebula " (spiral nebula is the obsolete term for spiral galaxies), and soon discovered that almost all such nebulae were receding from Earth. He did not grasp the cosmological implications of this fact, and indeed at the time it was highly controversial whether or not these nebulae were "island universes" outside our Milky Way . Ten years later, Alexander Friedmann ,

14852-477: The formation of structures in the universe ( stars , quasars , galaxies and galaxy groups and clusters ), also suggests that the density of matter in the universe is only 30% of the critical density. A 2011 survey, the WiggleZ galaxy survey of more than 200,000 galaxies, provided further evidence towards the existence of dark energy, although the exact physics behind it remains unknown. The WiggleZ survey from

15010-417: The fundamental constants in space or time. Scalar fields are predicted by the Standard Model of particle physics and string theory , but an analogous problem to the cosmological constant problem (or the problem of constructing models of cosmological inflation ) occurs: renormalization theory predicts that scalar fields should acquire large masses. The coincidence problem asks why the acceleration of

15168-404: The gravitational effects of an unknown dark matter surrounding galaxies. Most of the gravitational potential in the universe seems to be in this form, and the Big Bang models and various observations indicate that this excess gravitational potential is not created by baryonic matter , such as normal atoms. Measurements of the redshifts of supernovae indicate that the expansion of the universe

15326-416: The highest order, writing Principia Mathematica . In it contained a grand synthesis of the work of Copernicus, Galileo and Kepler; as well as Newton's theories of mechanics and gravitation, which held sway as worldviews until the early 20th century. Simultaneously, progress was also made in optics (in particular colour theory and the ancient science of geometrical optics ), courtesy of Newton, Descartes and

15484-455: The idea of energy (as well as its global conservation) by the inclusion of heat , electricity and magnetism , and then light . The laws of thermodynamics , and most importantly the introduction of the singular concept of entropy began to provide a macroscopic explanation for the properties of matter. Statistical mechanics (followed by statistical physics and Quantum statistical mechanics ) emerged as an offshoot of thermodynamics late in

15642-413: The interstellar space'. The mechanism was an example of fine-tuning , and it was later realized that Einstein's static universe would not be stable: local inhomogeneities would ultimately lead to either the runaway expansion or contraction of the universe. The equilibrium is unstable: if the universe expands slightly, then the expansion releases vacuum energy, which causes yet more expansion. Likewise,

15800-439: The known laws of physics , the models describe an increasingly concentrated cosmos preceded by a singularity in which space and time lose meaning (typically named "the Big Bang singularity"). Physics lacks a widely accepted theory of quantum gravity that can model the earliest conditions of the Big Bang. In 1964 the CMB was discovered, which convinced many cosmologists that the competing steady-state model of cosmic evolution

15958-409: The lambda-CDM model of cosmology, which uses the independent frameworks of quantum mechanics and general relativity. There are no easily testable models that would describe the situation prior to approximately 10 seconds. Understanding this earliest of eras in the history of the universe is one of the greatest unsolved problems in physics . English astronomer Fred Hoyle is credited with coining

16116-488: The largest possible deviation of the fine-structure constant over much of the age of the universe is of order 10. Also, general relativity has passed stringent tests on the scale of the Solar System and binary stars . The large-scale universe appears isotropic as viewed from Earth. If it is indeed isotropic, the cosmological principle can be derived from the simpler Copernican principle , which states that there

16274-675: The late 1920s. In the aftermath of World War 2, more progress brought much renewed interest in QFT, which had since the early efforts, stagnated. The same period also saw fresh attacks on the problems of superconductivity and phase transitions, as well as the first applications of QFT in the area of theoretical condensed matter. The 1960s and 70s saw the formulation of the Standard model of particle physics using QFT and progress in condensed matter physics (theoretical foundations of superconductivity and critical phenomena , among others ), in parallel to

16432-405: The light emitted from them has been shifted to longer wavelengths. This can be seen by taking a frequency spectrum of an object and matching the spectroscopic pattern of emission or absorption lines corresponding to atoms of the chemical elements interacting with the light. These redshifts are uniformly isotropic, distributed evenly among the observed objects in all directions. If the redshift

16590-471: The matter density is around 30% of critical. The large difference between these two supports a smooth component of dark energy making up the difference. Much more precise measurements from WMAP in 2003–2010 have continued to support the standard model and give more accurate measurements of the key parameters. The term "dark energy", echoing Fritz Zwicky 's "dark matter" from the 1930s, was coined by Michael S. Turner in 1998. High-precision measurements of

16748-694: The model under-predicted observations of large-scale galaxy clustering. These difficulties became stronger after the discovery of anisotropy in the cosmic microwave background by the COBE spacecraft in 1992, and several modified CDM models came under active study through the mid-1990s: these included the Lambda-CDM model and a mixed cold/hot dark matter model. The first direct evidence for dark energy came from supernova observations in 1998 of accelerated expansion in Riess et al. and in Perlmutter et al. , and

16906-428: The modified dynamics stems to what have been assigned to the presence of dark energy and dark matter. Dark energy could in principle interact not only with the rest of the dark sector, but also with ordinary matter. However, cosmology alone is not sufficient to effectively constrain the strength of the coupling between dark energy and baryons, so that other indirect techniques or laboratory searches have to be adopted. It

17064-475: The need for dark energy. There are many such theories, and research is ongoing. The measurement of the speed of gravity in the first gravitational wave measured by non-gravitational means ( GW170817 ) ruled out many modified gravity theories as explanations to dark energy. Astrophysicist Ethan Siegel states that, while such alternatives gain mainstream press coverage, almost all professional astrophysicists are confident that dark energy exists and that none of

17222-452: The notions of space and time would altogether fail to have any meaning at the beginning; they would only begin to have a sensible meaning when the original quantum had been divided into a sufficient number of quanta. If this suggestion is correct, the beginning of the world happened a little before the beginning of space and time. During the 1930s, other ideas were proposed as non-standard cosmologies to explain Hubble's observations, including

17380-481: The observational data by a more refined use of established theories. In this scenario, dark energy does not actually exist, and is merely a measurement artifact. For example, if we are located in an emptier-than-average region of space, the observed cosmic expansion rate could be mistaken for a variation in time, or acceleration. A different approach uses a cosmological extension of the equivalence principle to show how space might appear to be expanding more rapidly in

17538-459: The observational evidence, most notably from radio source counts , began to favor Big Bang over steady state. The discovery and confirmation of the CMB in 1964 secured the Big Bang as the best theory of the origin and evolution of the universe. In 1968 and 1970, Roger Penrose , Stephen Hawking , and George F. R. Ellis published papers where they showed that mathematical singularities were an inevitable initial condition of relativistic models of

17696-481: The opacity of the universe at early times. So our view cannot extend further backward in time, though the horizon recedes in space. If the expansion of the universe continues to accelerate, there is a future horizon as well. Some processes in the early universe occurred too slowly, compared to the expansion rate of the universe, to reach approximate thermodynamic equilibrium . Others were fast enough to reach thermalization . The parameter usually used to find out whether

17854-403: The original matter particles and none of their antiparticles . A similar process happened at about 1 second for electrons and positrons. After these annihilations, the remaining protons, neutrons and electrons were no longer moving relativistically and the energy density of the universe was dominated by photons (with a minor contribution from neutrinos ). A few minutes into the expansion, when

18012-579: The original paper. Another study questioning the essential assumption that the luminosity of Type Ia supernovae does not vary with stellar population age was also swiftly rebutted by other cosmologists. This theory was formulated by researchers of the University of Hawaiʻi at Mānoa in February 2023. The idea is that if one requires the Kerr metric (which describes rotating black holes) to asymptote to

18170-528: The other astronomical structures observable today. The details of this process depend on the amount and type of matter in the universe. The four possible types of matter are known as cold dark matter (CDM), warm dark matter , hot dark matter , and baryonic matter . The best measurements available, from the Wilkinson Microwave Anisotropy Probe (WMAP), show that the data is well-fit by a Lambda-CDM model in which dark matter

18328-463: The other forces, with only the electromagnetic force and weak nuclear force remaining unified. Inflation stopped locally at around 10 to 10 seconds, with the observable universe's volume having increased by a factor of at least 10. Reheating followed as the inflaton field decayed, until the universe obtained the temperatures required for the production of a quark–gluon plasma as well as all other elementary particles . Temperatures were so high that

18486-426: The past all the mass of the universe was concentrated into a single point, a "primeval atom" where and when the fabric of time and space came into existence. In the 1920s and 1930s, almost every major cosmologist preferred an eternal steady-state universe, and several complained that the beginning of time implied by the Big Bang imported religious concepts into physics; this objection was later repeated by supporters of

18644-511: The photon radiation . The recombination epoch began after about 379,000 years, when the electrons and nuclei combined into atoms (mostly hydrogen ), which were able to emit radiation. This relic radiation, which continued through space largely unimpeded, is known as the cosmic microwave background. After the recombination epoch, the slightly denser regions of the uniformly distributed matter gravitationally attracted nearby matter and thus grew even denser, forming gas clouds, stars, galaxies, and

18802-504: The picture becomes less speculative, since particle energies drop to values that can be attained in particle accelerators . At about 10 seconds, quarks and gluons combined to form baryons such as protons and neutrons . The small excess of quarks over antiquarks led to a small excess of baryons over antibaryons. The temperature was no longer high enough to create either new proton–antiproton or neutron–antineutron pairs. A mass annihilation immediately followed, leaving just one in 10 of

18960-486: The precise values of the Hubble Constant and the matter-density of the universe (before the discovery of dark energy, thought to be the key predictor for the eventual fate of the universe ). In the mid-1990s, observations of certain globular clusters appeared to indicate that they were about 15 billion years old, which conflicted with most then-current estimates of the age of the universe (and indeed with

19118-456: The predominance of matter over antimatter in the present universe. The universe continued to decrease in density and fall in temperature, hence the typical energy of each particle was decreasing. Symmetry-breaking phase transitions put the fundamental forces of physics and the parameters of elementary particles into their present form, with the electromagnetic force and weak nuclear force separating at about 10 seconds. After about 10 seconds,

19276-407: The present-day observable universe while dark matter and ordinary (baryonic) matter contribute 26% and 5%, respectively, and other components such as neutrinos and photons are nearly negligible. Dark energy's density is very low: 7 × 10  g/cm ( 6 × 10  J/m in mass-energy ), much less than the density of ordinary matter or dark matter within galaxies. However, it dominates

19434-456: The problem remains unresolved. Independently of its actual nature, dark energy would need to have a strong negative pressure to explain the observed acceleration of the expansion of the universe . According to general relativity, the pressure within a substance contributes to its gravitational attraction for other objects just as its mass density does. This happens because the physical quantity that causes matter to generate gravitational effects

19592-419: The process of becoming established and some proposed theories. It can include speculative sciences. This includes physics fields and physical theories presented in accordance with known evidence, and a body of associated predictions have been made according to that theory. Some fringe theories go on to become a widely accepted part of physics. Other fringe theories end up being disproven. Some fringe theories are

19750-400: The quintessence field has a density which closely tracks (but is less than) the radiation density until matter–radiation equality , which triggers quintessence to start behaving as dark energy, eventually dominating the universe. This naturally sets the low energy scale of the dark energy. In 2004, when scientists fit the evolution of dark energy with the cosmological data, they found that

19908-426: The random motions of particles were at relativistic speeds , and particle–antiparticle pairs of all kinds were being continuously created and destroyed in collisions. At some point, an unknown reaction called baryogenesis violated the conservation of baryon number , leading to a very small excess of quarks and leptons over antiquarks and antileptons—of the order of one part in 30 million. This resulted in

20066-557: The remaining 70%. The Wilkinson Microwave Anisotropy Probe (WMAP) spacecraft seven-year analysis estimated a universe made up of 72.8% dark energy, 22.7% dark matter, and 4.5% ordinary matter. Work done in 2013 based on the Planck spacecraft observations of the cosmic microwave background gave a more accurate estimate of 68.3% dark energy, 26.8% dark matter, and 4.9% ordinary matter. Accelerated cosmic expansion causes gravitational potential wells and hills to flatten as photons pass through them, producing cold spots and hot spots on

20224-607: The rise of medieval universities , the only acknowledged intellectual disciplines were the seven liberal arts of the Trivium like grammar , logic , and rhetoric and of the Quadrivium like arithmetic , geometry , music and astronomy . During the Middle Ages and Renaissance , the concept of experimental science, the counterpoint to theory, began with scholars such as Ibn al-Haytham and Francis Bacon . As

20382-417: The scale factor is caused by the potential energy of a dynamical field , referred to as quintessence field. Quintessence differs from the cosmological constant in that it can vary in space and time. In order for it not to clump and form structure like matter, the field must be very light so that it has a large Compton wavelength . In the simplest scenarios, the quintessence field has a canonical kinetic term,

20540-407: The section Dark energy § Theories of dark energy ). The " cosmological constant " is a constant term that can be added to Einstein field equations of general relativity . If considered as a "source term" in the field equation, it can be viewed as equivalent to the mass of empty space (which conceptually could be either positive or negative), or " vacuum energy ". The cosmological constant

20698-406: The singularity. In some proposals, such as the emergent Universe models, the singularity is replaced by another cosmological epoch. A different approach identifies the initial singularity as a singularity predicted by some models of the Big Bang theory to have existed before the Big Bang event. This primordial singularity is itself sometimes called "the Big Bang", but the term can also refer to

20856-494: The size of the observable universe , the Hubble redshift can be thought of as the Doppler shift corresponding to the recession velocity v {\displaystyle v} . For distances comparable to the size of the observable universe, the attribution of the cosmological redshift becomes more ambiguous, although its interpretation as a kinematic Doppler shift remains the most natural one. An unexplained discrepancy with

21014-428: The statistical methods employed were flawed. A laboratory direct detection attempt failed to detect any force associated with dark energy. Observational skepticism explanations of dark energy have generally not gained much traction among cosmologists. For example, a paper that suggested the anisotropy of the local Universe has been misrepresented as dark energy was quickly countered by another paper claiming errors in

21172-402: The steady-state theory. This perception was enhanced by the fact that the originator of the Big Bang concept, Lemaître, was a Roman Catholic priest. Arthur Eddington agreed with Aristotle that the universe did not have a beginning in time, viz ., that matter is eternal . A beginning in time was "repugnant" to him. Lemaître, however, disagreed: If the world has begun with a single quantum ,

21330-484: The study of physics which include scientific approaches, means for determining the validity of models and new types of reasoning used to arrive at the theory. However, some proposed theories include theories that have been around for decades and have eluded methods of discovery and testing. Proposed theories can include fringe theories in the process of becoming established (and, sometimes, gaining wider acceptance). Proposed theories usually have not been tested. In addition to

21488-445: The temperature was about a billion kelvin and the density of matter in the universe was comparable to the current density of Earth's atmosphere, neutrons combined with protons to form the universe's deuterium and helium nuclei in a process called Big Bang nucleosynthesis (BBN). Most protons remained uncombined as hydrogen nuclei. As the universe cooled, the rest energy density of matter came to gravitationally dominate that of

21646-458: The term "Big Bang" during a talk for a March 1949 BBC Radio broadcast, saying: "These theories were based on the hypothesis that all the matter in the universe was created in one big bang at a particular time in the remote past." However, it did not catch on until the 1970s. It is popularly reported that Hoyle, who favored an alternative " steady-state " cosmological model, intended this to be pejorative, but Hoyle explicitly denied this and said it

21804-472: The theories like those listed below, there are also different interpretations of quantum mechanics , which may or may not be considered different theories since it is debatable whether they yield different predictions for physical experiments, even in principle. For example, AdS/CFT correspondence , Chern–Simons theory , graviton , magnetic monopole , string theory , theory of everything . Fringe theories include any new area of scientific endeavor in

21962-451: The total energy density of the present day universe is in this form. When the universe was very young it was likely infused with dark energy, but with everything closer together, gravity predominated, braking the expansion. Eventually, after billions of years of expansion, the declining density of matter relative to the density of dark energy allowed the expansion of the universe to begin to accelerate. Dark energy in its simplest formulation

22120-458: The ultimate result will be that galaxies outside the Local Group will have a line-of-sight velocity that continually increases with time, eventually far exceeding the speed of light. This is not a violation of special relativity because the notion of "velocity" used here is different from that of velocity in a local inertial frame of reference , which is still constrained to be less than

22278-414: The universe has no overall geometric curvature due to gravitational influence. Microscopic quantum fluctuations that occurred because of Heisenberg's uncertainty principle were "frozen in" by inflation, becoming amplified into the seeds that would later form the large-scale structure of the universe. At a time around 10 seconds, the electroweak epoch begins when the strong nuclear force separates from

22436-422: The universe has a finite age, and light travels at a finite speed, there may be events in the past whose light has not yet had time to reach earth. This places a limit or a past horizon on the most distant objects that can be observed. Conversely, because space is expanding, and more distant objects are receding ever more quickly, light emitted by us today may never "catch up" to very distant objects. This defines

22594-488: The universe is uniformly expanding everywhere. This cosmic expansion was predicted from general relativity by Friedmann in 1922 and Lemaître in 1927, well before Hubble made his 1929 analysis and observations, and it remains the cornerstone of the Big Bang model as developed by Friedmann, Lemaître, Robertson, and Walker. The theory requires the relation v = H D {\displaystyle v=HD} to hold at all times, where D {\displaystyle D}

22752-403: The universe's mass–energy content because it is uniform across space. The first observational evidence for dark energy's existence came from measurements of supernovae . Type Ia supernovae have constant luminosity, which means that they can be used as accurate distance measures. Comparing this distance to the redshift (which measures the speed at which the supernova is receding) shows that

22910-438: The universe. Further, observations made by Edwin Hubble in 1929 showed that the universe appears to be expanding and is not static. Einstein reportedly referred to his failure to predict the idea of a dynamic universe, in contrast to a static universe, as his greatest blunder. Alan Guth and Alexei Starobinsky proposed in 1980 that a negative pressure field, similar in concept to dark energy, could drive cosmic inflation in

23068-438: The vacuum energy should exert a gravitational force. Hence, the vacuum energy is expected to contribute to the cosmological constant , which in turn impinges on the accelerated expansion of the universe . However, the cosmological constant problem asserts that there is a huge disagreement between the observed values of vacuum energy density and the theoretical large value of zero-point energy obtained by quantum field theory ;

23226-448: The validity of the theory are the expansion of the universe according to Hubble's law (as indicated by the redshifts of galaxies), discovery and measurement of the cosmic microwave background and the relative abundances of light elements produced by Big Bang nucleosynthesis (BBN). More recent evidence includes observations of galaxy formation and evolution , and the distribution of large-scale cosmic structures . These are sometimes called

23384-475: The very early universe. Inflation postulates that some repulsive force, qualitatively similar to dark energy, resulted in an enormous and exponential expansion of the universe slightly after the Big Bang . Such expansion is an essential feature of most current models of the Big Bang. However, inflation must have occurred at a much higher (negative) energy density than the dark energy we observe today, and inflation

23542-539: The vision provided by pure mathematical systems can provide clues to how a physical system might be modeled; e.g., the notion, due to Riemann and others, that space itself might be curved. Theoretical problems that need computational investigation are often the concern of computational physics . Theoretical advances may consist in setting aside old, incorrect paradigms (e.g., aether theory of light propagation, caloric theory of heat, burning consisting of evolving phlogiston , or astronomical bodies revolving around

23700-409: The voids surrounding our local cluster. While weak, such effects considered cumulatively over billions of years could become significant, creating the illusion of cosmic acceleration, and making it appear as if we live in a Hubble bubble . Yet other possibilities are that the accelerated expansion of the universe is an illusion caused by the relative motion of us to the rest of the universe, or that

23858-530: The works of these men (alongside Galileo's) can perhaps be considered to constitute the Scientific Revolution. The great push toward the modern concept of explanation started with Galileo , one of the few physicists who was both a consummate theoretician and a great experimentalist . The analytic geometry and mechanics of Descartes were incorporated into the calculus and mechanics of Isaac Newton , another theoretician/experimentalist of

24016-420: Was falsified , since the Big Bang models predict a uniform background radiation caused by high temperatures and densities in the distant past. A wide range of empirical evidence strongly favors the Big Bang event, which is now essentially universally accepted. Detailed measurements of the expansion rate of the universe place the Big Bang singularity at an estimated 13.787 ± 0.020   billion years ago, which

24174-472: Was Lemaître's Big Bang theory, advocated and developed by George Gamow , who introduced BBN and whose associates, Ralph Alpher and Robert Herman , predicted the CMB. Ironically, it was Hoyle who coined the phrase that came to be applied to Lemaître's theory, referring to it as "this big bang idea" during a BBC Radio broadcast in March 1949. For a while, support was split between these two theories. Eventually,

24332-532: Was briefly theorized in the early 2020s that excess observed in the XENON1T detector in Italy may have been caused by a chameleon model of dark energy, but further experiments disproved this possibility. The density of dark energy might have varied in time during the history of the universe. Modern observational data allows us to estimate the present density of dark energy. Using baryon acoustic oscillations , it

24490-419: Was first proposed by Einstein as a mechanism to obtain a solution to the gravitational field equation that would lead to a static universe, effectively using dark energy to balance gravity. Einstein gave the cosmological constant the symbol Λ (capital lambda). Einstein stated that the cosmological constant required that 'empty space takes the role of gravitating negative masses which are distributed all over

24648-422: Was given a kinematic explanation by general relativity . Quantum mechanics led to an understanding of blackbody radiation (which indeed, was an original motivation for the theory) and of anomalies in the specific heats of solids — and finally to an understanding of the internal structures of atoms and molecules . Quantum mechanics soon gave way to the formulation of quantum field theory (QFT), begun in

24806-405: Was just a striking image meant to highlight the difference between the two models. Helge Kragh writes that the evidence for the claim that it was meant as a pejorative is "unconvincing", and mentions a number of indications that it was not a pejorative. The term itself has been argued to be a misnomer because it evokes an explosion. The argument is that whereas an explosion suggests expansion into

24964-423: Was very rapidly expanding and cooling. The period up to 10 seconds into the expansion, the Planck epoch , was a phase in which the four fundamental forces —the electromagnetic force , the strong nuclear force , the weak nuclear force , and the gravitational force , were unified as one. In this stage, the characteristic scale length of the universe was the Planck length , 1.6 × 10 m , and consequently had

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