In physical cosmology, the **age of the universe** is the time elapsed since the Big Bang. The current measurement of the age of the universe is 13.799±0.021 billion (10^{9}) years within the Lambda-CDM concordance model.^{[1]}^{[2]} The uncertainty has been narrowed down to 21 million years, based on a number of projects that all give extremely close figures for the age. These include studies of the microwave background radiation, and measurements by the *Planck* spacecraft, the Wilkinson Microwave Anisotropy Probe and other probes. Measurements of the cosmic background radiation give the cooling time of the universe since the Big Bang,^{[3]} and measurements of the expansion rate of the universe can be used to calculate its approximate age by extrapolating backwards in time.

The Lambda-CDM concordance model describes the evolution of the universe from a very uniform, hot, dense primordial state to its present state over a span of about 13.8 billion years^{[4]} of cosmological time. This model is well understood theoretically and strongly supported by recent high-precision astronomical observations such as WMAP. In contrast, theories of the origin of the primordial state remain very speculative. If one extrapolates the Lambda-CDM model backward from the earliest well-understood state, it quickly (within a small fraction of a second) reaches a singularity. This is known as the "initial singularity" or the "Big Bang singularity". This singularity is not understood as having a physical significance in the usual sense, but it is convenient to quote times measured "since the Big Bang" even though they do not correspond to a physically measurable time. For example, "10^{−6} seconds after the Big Bang" is a well-defined era in the universe's evolution. If one referred to the same era as "13.8 billion years minus 10^{−6} seconds ago", the precision of the meaning would be lost because the minuscule latter time interval is eclipsed by uncertainty in the former.

Though the universe might in theory have a longer history, the International Astronomical Union^{[5]} presently use "age of the universe" to mean the duration of the Lambda-CDM expansion, or equivalently the elapsed time since the Big Bang in the current observable universe.

Since the universe must be at least as old as the oldest things in it, there are a number of observations which put a lower limit on the age of the universe; these include the temperature of the coolest white dwarfs, which gradually cool as they age, and the dimmest turnoff point of main sequence stars in clusters (lower-mass stars spend a greater amount of time on the main sequence, so the lowest-mass stars that have evolved off of the main sequence set a minimum age).

The problem of determining the age of the universe is closely tied to the problem of determining the values of the cosmological parameters. Today this is largely carried out in the context of the ΛCDM model, where the universe is assumed to contain normal (baryonic) matter, cold dark matter, radiation (including both photons and neutrinos), and a cosmological constant. The fractional contribution of each to the current energy density of the universe is given by the density parameters Ω_{m}, Ω_{r}, and Ω_{Λ}. The full ΛCDM model is described by a number of other parameters, but for the purpose of computing its age these three, along with the Hubble parameter , are the most important.

If one has accurate measurements of these parameters, then the age of the universe can be determined by using the Friedmann equation. This equation relates the rate of change in the scale factor *a*(*t*) to the matter content of the universe. Turning this relation around, we can calculate the change in time per change in scale factor and thus calculate the total age of the universe by integrating this formula. The age *t*_{0} is then given by an expression of the form

where is the Hubble parameter and the function *F* depends only on the fractional contribution to the universe's energy content that comes from various components. The first observation that one can make from this formula is that it is the Hubble parameter that controls that age of the universe, with a correction arising from the matter and energy content. So a rough estimate of the age of the universe comes from the Hubble time, the inverse of the Hubble parameter. With a value for around 68 km/s/Mpc, the Hubble time evaluates to = 14.4 billion years.^{[6]}

To get a more accurate number, the correction factor *F* must be computed. In general this must be done numerically, and the results for a range of cosmological parameter values are shown in the figure. For the Planck values (Ω_{m}, Ω_{Λ}) = (0.3086, 0.6914), shown by the box in the upper left corner of the figure, this correction factor is about *F* = 0.956. For a flat universe without any cosmological constant, shown by the star in the lower right corner, *F* = ^{2}⁄_{3} is much smaller and thus the universe is younger for a fixed value of the Hubble parameter. To make this figure, Ω_{r} is held constant (roughly equivalent to holding the CMB temperature constant) and the curvature density parameter is fixed by the value of the other three.

Apart from the Planck satellite, the Wilkinson Microwave Anisotropy Probe (WMAP) was instrumental in establishing an accurate age of the universe, though other measurements must be folded in to gain an accurate number. CMB measurements are very good at constraining the matter content Ω_{m}^{[7]} and curvature parameter Ω_{k}.^{[8]} It is not as sensitive to Ω_{Λ} directly,^{[8]} partly because the cosmological constant becomes important only at low redshift. The most accurate determinations of the Hubble parameter *H*_{0} come from Type Ia supernovae. Combining these measurements leads to the generally accepted value for the age of the universe quoted above.

The cosmological constant makes the universe "older" for fixed values of the other parameters. This is significant, since before the cosmological constant became generally accepted, the Big Bang model had difficulty explaining why globular clusters in the Milky Way appeared to be far older than the age of the universe as calculated from the Hubble parameter and a matter-only universe.^{[9]}^{[10]} Introducing the cosmological constant allows the universe to be older than these clusters, as well as explaining other features that the matter-only cosmological model could not.^{[11]}

NASA's Wilkinson Microwave Anisotropy Probe (WMAP) project's nine-year data release in 2012 estimated the age of the universe to be (13.772±0.059)×10^{9} years (13.772 billion years, with an uncertainty of plus or minus 59 million years).^{[3]}

However, this age is based on the assumption that the project's underlying model is correct; other methods of estimating the age of the universe could give different ages. Assuming an extra background of relativistic particles, for example, can enlarge the error bars of the WMAP constraint by one order of magnitude.^{[12]}

This measurement is made by using the location of the first acoustic peak in the microwave background power spectrum to determine the size of the decoupling surface (size of the universe at the time of recombination). The light travel time to this surface (depending on the geometry used) yields a reliable age for the universe. Assuming the validity of the models used to determine this age, the residual accuracy yields a margin of error near one percent.^{[13]}

In 2015, the Planck Collaboration estimated the age of the universe to be 13.813±0.038 billion years, slightly higher but within the uncertainties of the earlier number derived from the WMAP data. By combining the Planck data with external data, the best combined estimate of the age of the universe is (13.799±0.021)×10^{9} years old.^{[1]}^{[2]}

Parameter | Symbol | TT+lowP 68% limits |
TT+lowP +lensing 68% limits |
TT+lowP +lensing+ext 68% limits |
TT,TE,EE+lowP 68% limits |
TT,TE,EE+lowP +lensing 68% limits |
TT,TE,EE+lowP +lensing+ext 68% limits |
---|---|---|---|---|---|---|---|

Age of the universe (Ga) |
13.813±0.038 | 13.799±0.038 | 13.796±0.029 | 13.813±0.026 | 13.807±0.026 | 13.799±0.021 | |

Hubble constant ( ^{km}⁄_{Mpc•s} ) |
67.31±0.96 | 67.81±0.92 | 67.90±0.55 | 67.27±0.66 | 67.51±0.64 | 67.74±0.46 |

Calculating the age of the universe is accurate only if the assumptions built into the models being used to estimate it are also accurate. This is referred to as strong priors and essentially involves stripping the potential errors in other parts of the model to render the accuracy of actual observational data directly into the concluded result. Although this is not a valid procedure in all contexts (as noted in the accompanying caveat: "based on the fact we have assumed the underlying model we used is correct"), the age given is thus accurate to the specified error (since this error represents the error in the instrument used to gather the raw data input into the model).

The age of the universe based on the best fit to Planck 2015 data alone is 13.813±0.038 billion years (the estimate of 13.799±0.021 billion years uses Gaussian priors based on earlier estimates from other studies to determine the combined uncertainty). This number represents an accurate "direct" measurement of the age of the universe (other methods typically involve Hubble's law and the age of the oldest stars in globular clusters, etc.). It is possible to use different methods for determining the same parameter (in this case – the age of the universe) and arrive at different answers with no overlap in the "errors". To best avoid the problem, it is common to show two sets of uncertainties; one related to the actual measurement and the other related to the systematic errors of the model being used.

An important component to the analysis of data used to determine the age of the universe (e.g. from Planck) therefore is to use a Bayesian statistical analysis, which normalizes the results based upon the priors (i.e. the model).^{[13]} This quantifies any uncertainty in the accuracy of a measurement due to a particular model used.^{[14]}^{[15]}

In the 18th century, the concept that the age of the Earth was millions, if not billions, of years began to appear. However, most scientists throughout the 19th century and into the first decades of the 20th century presumed that the universe itself was Steady State and eternal, with maybe stars coming and going but no changes occurring at the largest scale known at the time.

The first scientific theories indicating that the age of the universe might be finite were the studies of thermodynamics, formalized in the mid-19th century. The concept of entropy dictates that if the universe (or any other closed system) were infinitely old, then everything inside would be at the same temperature, and thus there would be no stars and no life. No scientific explanation for this contradiction was put forth at the time.

In 1915 Albert Einstein published the theory of general relativity^{[16]} and in 1917 constructed the first cosmological model based on his theory. In order to remain consistent with a steady state universe, Einstein added what was later called a cosmological constant to his equations. However, already in 1922, also using Einstein's theory, Alexander Friedmann, and independently five years later Georges Lemaître, showed that the universe cannot be static and must be either expanding or contracting. Einstein's model of a static universe was in addition proved unstable by Arthur Eddington.

The first direct observational hint that the universe has a finite age came from the observations of 'recession velocities', mostly by Vesto Slipher, combined with distances to the 'nebulae' (galaxies) by Edwin Hubble in a work published in 1929.^{[17]} Earlier in the 20th century, Hubble and others resolved individual stars within certain nebulae, thus determining that they were galaxies, similar to, but external to, our Milky Way Galaxy. In addition, these galaxies were very large and very far away. Spectra taken of these distant galaxies showed a red shift in their spectral lines presumably caused by the Doppler effect, thus indicating that these galaxies were moving away from the Earth. In addition, the farther away these galaxies seemed to be (the dimmer they appeared to us) the greater was their redshift, and thus the faster they seemed to be moving away. This was the first direct evidence that the universe is not static but expanding. The first estimate of the age of the universe came from the calculation of when all of the objects must have started speeding out from the same point. Hubble's initial value for the universe's age was very low, as the galaxies were assumed to be much closer than later observations found them to be.

The first reasonably accurate measurement of the rate of expansion of the universe, a numerical value now known as the Hubble constant, was made in 1958 by astronomer Allan Sandage.^{[18]} His measured value for the Hubble constant came very close to the value range generally accepted today.

However Sandage, like Einstein, did not believe his own results at the time of discovery. His value for the age of the universe was too short to reconcile with the 25-billion-year age estimated at that time for the oldest known stars. Sandage and other astronomers repeated these measurements numerous times, attempting to reduce the Hubble constant and thus increase the resulting age for the universe. Sandage even proposed new theories of cosmogony to explain this discrepancy. This issue was finally resolved by improvements in the theoretical models used for estimating the ages of stars. As of 2013, using the latest models for stellar evolution, the estimated age of the oldest known star is 14.46±0.8 billion years.^{[19]}

The discovery of microwave cosmic background radiation announced in 1965^{[20]} finally brought an effective end to the remaining scientific uncertainty over the expanding universe. It was a chance result from work by two teams less than 60 miles apart. In 1964, Arno Penzias and Robert Wilson were trying to detect radio wave echoes with a supersensitive antenna. The antenna persistently detected a low, steady, mysterious noise in the microwave region that was evenly spread over the sky, and was present day and night. After testing, they became certain that the signal did not come from the Earth, the Sun, or our galaxy, but from outside our own galaxy, but could not explain it. At the same time another team, Robert H. Dicke, Jim Peebles, and David Wilkinson, were attempting to detect low level noise which might be left over from the Big Bang and could prove whether the Big Bang theory was correct. The two teams realized that the detected noise was in fact radiation left over from the Big Bang, and that this was strong evidence that the theory was correct. Since then, a great deal of other evidence has strengthened and confirmed this conclusion, and refined the estimated age of the universe to its current figure.

The space probes WMAP, launched in 2001, and Planck, launched in 2009, produced data that determines the Hubble constant and the age of the universe independent of galaxy distances, removing the largest source of error.^{[13]}

- Age of the Earth
- Timeline of the far future: expected remaining lifetime of the Earth; our solar system; the universe
- Anthropic principle
- Cosmic Calendar (age of universe scaled to a single year)
- Cosmology
- Dark Ages Radio Explorer
- Hubble Deep Field
- Illustris project
- Metric expansion of space
- Multiverse
- Observable universe
- Red shift observations in astronomy
- Static universe
*The First Three Minutes*(1977 book by Steven Weinberg).

- ^
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^{a}^{b}Lawrence, C. R. (18 March 2015). "Planck 2015 Results" (PDF). Archived from the original (PDF) on 2016-11-24. Retrieved 24 November 2016. - ^
^{a}^{b}Bennett, C.L.; et al. (2013). "Nine-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Final Maps and Results".*The Astrophysical Journal Supplement Series*.**208**(2): 20. arXiv:1212.5225. Bibcode:2013ApJS..208...20B. doi:10.1088/0067-0049/208/2/20. **^**"Cosmic Detectives". European Space Agency. 2 April 2013. Retrieved 2013-04-15.**^**Chang, K. (9 March 2008). "Gauging Age of Universe Becomes More Precise".*The New York Times*.**^**Liddle, A. R. (2003).*An Introduction to Modern Cosmology*(2nd ed.). Wiley. p. 57. ISBN 978-0-470-84835-7.**^**Hu, W. "Animation: Matter Content Sensitivity. The matter-radiation ratio is raised while keeping all other parameters fixed". University of Chicago. Archived from the original on 23 February 2008. Retrieved 2008-02-23.- ^
^{a}^{b}Hu, W. "Animation: Angular diameter distance scaling with curvature and lambda". University of Chicago. Archived from the original on 23 February 2008. Retrieved 2008-02-23. **^**"Globular Star Clusters". SEDS. 1 July 2011. Archived from the original on 24 February 2008. Retrieved 2013-07-19.**^**Iskander, E. (11 January 2006). "Independent age estimates". University of British Columbia. Archived from the original on 6 March 2008. Retrieved 2008-02-23.**^**Ostriker, J. P.; Steinhardt, P. J. (1995). "Cosmic Concordance". arXiv:astro-ph/9505066.**^**de Bernardis, F.; Melchiorri, A.; Verde, L.; Jimenez, R. (2008). "The Cosmic Neutrino Background and the Age of the Universe".*Journal of Cosmology and Astroparticle Physics*.**2008**(3): 20. arXiv:0707.4170. Bibcode:2008JCAP...03..020D. doi:10.1088/1475-7516/2008/03/020.- ^
^{a}^{b}^{c}Spergel, D. N.; et al. (2003). "First-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Determination of Cosmological Parameters".*The Astrophysical Journal Supplement Series*.**148**(1): 175–194. arXiv:astro-ph/0302209. Bibcode:2003ApJS..148..175S. doi:10.1086/377226. **^**Loredo, T. J. (1992). "The Promise of Bayesian Inference for Astrophysics" (PDF). In Feigelson, E. D.; Babu, G. J.*Statistical Challenges in Modern Astronomy*. Springer-Verlag. pp. 275–297. Bibcode:1992scma.conf..275L. doi:10.1007/978-1-4613-9290-3_31. ISBN 978-1-4613-9292-7.**^**Colistete, R.; Fabris, J. C.; Concalves, S. V. B. (2005). "Bayesian Statistics and Parameter Constraints on the Generalized Chaplygin Gas Model Using SNe ia Data".*International Journal of Modern Physics D*.**14**(5): 775–796. arXiv:astro-ph/0409245. Bibcode:2005IJMPD..14..775C. doi:10.1142/S0218271805006729.**^**Einstein, A. (1915). "Zur allgemeinen Relativitätstheorie".*Sitzungsberichte der Königlich Preußischen Akademie der Wissenschaften*(in German): 778–786. Bibcode:1915SPAW.......778E.**^**Hubble, E. (1929). "A relation between distance and radial velocity among extra-galactic nebulae" (PDF).*Proceedings of the National Academy of Sciences*.**15**(3): 168–173. Bibcode:1929PNAS...15..168H. doi:10.1073/pnas.15.3.168. PMC 522427. PMID 16577160.**^**Sandage, A. R. (1958). "Current Problems in the Extragalactic Distance Scale".*The Astrophysical Journal*.**127**(3): 513–526. Bibcode:1958ApJ...127..513S. doi:10.1086/146483.**^**Bond, H. E.; Nelan, E. P.; Vandenberg, D. A.; Schaefer, G. H.; Harmer, D. (2013). "HD 140283: A Star in the Solar Neighborhood that Formed Shortly After the Big Bang".*The Astrophysical Journal*.**765**(12): L12. arXiv:1302.3180. Bibcode:2013ApJ...765L..12B. doi:10.1088/2041-8205/765/1/L12.**^**Penzias, A. A.; Wilson, R .W. (1965). "A Measurement of Excess Antenna Temperature at 4080 Mc/s".*The Astrophysical Journal*.**142**: 419–421. Bibcode:1965ApJ...142..419P. doi:10.1086/148307.

- Ned Wright's Cosmology Tutorial
- Wright, Edward L. (2 July 2005). "Age of the Universe".
- Wayne Hu's cosmological parameter animations
- Ostriker; Steinhardt (1995). "Cosmic Concordance". arXiv:astro-ph/9505066.
- SEDS page on "Globular Star Clusters"
- Douglas Scott "Independent Age Estimates"
- KryssTal "The Scale of the Universe" Space and Time scaled for the beginner.
- iCosmos: Cosmology Calculator (With Graph Generation )
- The Expanding Universe (American Institute of Physics)

The accelerating expansion of the universe is the observation that the expansion of the universe is such that the velocity at which a distant galaxy is receding from the observer is continuously increasing with time.The accelerated expansion was discovered during 1998, by two independent projects, the Supernova Cosmology Project and the High-Z Supernova Search Team, which both used distant type Ia supernovae to measure the acceleration. The idea was that as type 1a supernovae have almost the same intrinsic brightness (a standard candle), and since objects that are further away appear dimmer, we can use the observed brightness of these supernovae to measure the distance to them. The distance can then be compared to the supernovae's cosmological redshift, which measures how much the universe has expanded since the supernova occurred. The unexpected result was that objects in the universe are moving away from another at an accelerated rate. Cosmologists at the time expected that recession velocity would always be decelerating to the gravitational attraction of the matter in the universe. Three members of these two groups have subsequently been awarded Nobel Prizes for their discovery. Confirmatory evidence has been found in baryon acoustic oscillations, and in analyses of the clustering of galaxies.

The accelerated expansion of the universe is thought to have begun since the universe entered its dark-energy-dominated era roughly 5 billion years ago.

Within the framework of general relativity, an accelerated expansion can be accounted for by a positive value of the cosmological constant Λ, equivalent to the presence of a positive vacuum energy, dubbed "dark energy". While there are alternative possible explanations, the description assuming dark energy (positive Λ) is used in the current standard model of cosmology, which also includes cold dark matter (CDM) and is known as the Lambda-CDM model.

Allan SandageAllan Rex Sandage (June 18, 1926 – November 13, 2010) was an American astronomer. He was Staff Member Emeritus with the Carnegie Observatories in Pasadena, California. He determined the first reasonably accurate values for the Hubble constant and the age of the universe.

Black dwarfA black dwarf is a theoretical stellar remnant, specifically a white dwarf that has cooled sufficiently that it no longer emits significant heat or light. Because the time required for a white dwarf to reach this state is calculated to be longer than the current age of the universe (13.8 billion years), no black dwarfs are expected to exist in the universe now, and the temperature of the coolest white dwarfs is one observational limit on the age of the universe.The name "black dwarf" has also been applied to substellar objects that do not have sufficient mass, less than approximately 0.08 M☉, to maintain hydrogen-burning nuclear fusion. These objects are now generally called brown dwarfs, a term coined in the 1970s. Black dwarfs should not be confused with black holes, black stars, or neutron stars.

Cosmological decadeA cosmological decade (CÐ) is a division of the lifetime of the cosmos. The divisions are logarithmic in size, with base 10. Each successive cosmological decade represents a ten-fold increase in the total age of the universe.

Exa-Exa is a decimal unit prefix in the metric system denoting 1018 or 1000000000000000000. It was added as an SI prefix to the International System of Units (SI) in 1975, and has the unit symbol E.

Exa comes from the Ancient Greek ἕξ héx, used as a prefix ἑξά- hexá-, meaning six (like hexa-), because it is equal to 10006.

Examples:

The total storage needed by Google Mail as of April 2012, ignoring backups and compression, is more than an exabyte (10,240 megabytes of storage per user multiplied by an estimated 260 million users).

1 EeV = 1018 electronvolts = 0.1602 joule

United States electric energy consumption is about 15 exajoule per year.

1 exasecond is approximately 32 billion years

1 exametre is approximately 110 light years

0.43 Es ≈ the approximate age of the Universe

1.6 Em—172 ± 12.5 light years—Diameter of Omega Centauri (one of the largest known globular clusters, perhaps containing over a million stars)

23.6 exahashes/s is the calculation rate of the Bitcoin network ≈ 23600000000000000000 hashes per second (Mar 2018)

Graphical timeline of the universeThis more than 20-billion-year timeline of our universe shows the best estimates of major events from the universe's beginning to anticipated future events. Zero on the scale is the present day. A large step on the scale is one billion years; a small step, one hundred million years. The past is denoted by a minus sign: e.g., the oldest rock on Earth was formed about four billion years ago and this is marked at -4e+09 years, where 4e+09 represents 4 times 10 to the power of 9. The "Big Bang" event most likely happened 13.8 billion years ago; see age of the universe.

HE 1327-2326HE1327-2326, discovered in 2005 by Anna Frebel and collaborators, was the star with the lowest known iron abundance until SMSS J031300.36-670839.3 was discovered. The star is a member of Population II stars, with a solar-standardised iron to hydrogen index (Fe:H), or metallicity, of −5.6. The scale being logarithmic, this number indicates that its iron content is 1/400,000 that of the Earth's sun. However, it has a carbon abundance of roughly one-tenth solar ([C/H] = −1.0), and it is not known how these two abundances can have been produced/exist simultaneously. Discovered by the Hamburg/ESO survey for metal-poor stars, it was probably formed during an age of the universe when the metal content was much lower. It has been speculated that this star is part of the second generation, born out of the gas clouds which were imbued with elements such as carbon by the primordial Population III stars.

Heat death paradoxFormulated in 1862 by Lord Kelvin, Hermann von Helmholtz and William John Macquorn Rankine, the heat death paradox, also known as Clausius's paradox and thermodynamic paradox, is a reductio ad absurdum argument that uses thermodynamics to show the impossibility of an infinitely old universe.

Assuming that the universe is eternal, a question arises: How is it that thermodynamic equilibrium has not already been achieved?

This paradox is based upon the classical model of the universe in which the universe is eternal. Clausius's paradox is a paradox of paradigm. It was necessary to amend the fundamental ideas about the universe, which brought about the change of the paradigm. The paradox was solved when the paradigm was changed.

The paradox was based upon the rigid mechanical point of view of the second law of thermodynamics postulated by Rudolf Clausius according to which heat can only be transferred from a warmer to a colder object. If the universe was eternal, as claimed in the classical stationary model of the universe, it should already be cold.Any hot object transfers heat to its cooler surroundings, until everything is at the same temperature. For two objects at the same temperature as much heat flows from one body as flows from the other, and the net effect is no change. If the universe were infinitely old, there must have been enough time for the stars to cool and warm their surroundings. Everywhere should therefore be at the same temperature and there should either be no stars, or everything should be as hot as stars.

Since there are stars and the universe is not in thermal equilibrium it cannot be infinitely old.

The paradox does not arise in Big Bang or steady state cosmology. In Big Bang cosmology, the current age of the universe is not old enough to have reached equilibrium; while in a steady state system, sufficient hydrogen is replenished or regenerated continuously to allow for a constant average density and preventing stars from running down.

Hubble's law**Hubble's law**, also known as the **Hubble–Lemaître law**, is the observation in physical cosmology that:

Hubble's law is considered the first observational basis for the expansion of the universe and today serves as one of the pieces of evidence most often cited in support of the Big Bang model.
The motion of astronomical objects due solely to this expansion is known as the **Hubble flow**.

Although widely attributed to Edwin Hubble, the law was first derived from the general relativity equations in 1922 by Alexander Friedmann. Friedmann published a set of equations, now known as the Friedmann equations, showing that the universe might expand, and presenting the expansion speed if this was the case. Then Georges Lemaître, in a 1927 article, proposed the expansion of the universe and suggested an estimated value of the rate of expansion, which when corrected by Hubble became known as the **Hubble constant**. However, though the Hubble constant is roughly constant in the velocity-distance space at this moment in time, the **Hubble parameter** , which the Hubble constant is the current value of, changes with time, so the term 'constant' is sometimes thought of as somewhat of a misnomer. Moreover, two years later Edwin Hubble confirmed the existence of cosmic expansion, and determined a more accurate value for the constant that now bears his name.
Hubble inferred the recession velocity of the objects from their redshifts, many of which were earlier measured and related to velocity by Vesto Slipher in 1917. In October 2018, scientists presented a new third way (two earlier methods gave problematic results that do not agree), using information from gravitational wave events (especially those involving the merger of neutron stars, like GW170817), of determining the Hubble Constant, essential in establishing the exact rate of expansion of the universe.

The law is often expressed by the equation *v* = *H*_{0}*D*, with *H*_{0} the constant of proportionality—Hubble constant—between the "proper distance" *D* to a galaxy, which can change over time, unlike the comoving distance, and its velocity *v*, i.e. the derivative of proper distance with respect to cosmological time coordinate. (See *uses of the proper distance* for some discussion of the subtleties of this definition of 'velocity'.) Also, the SI unit of *H*_{0} is s^{−1}, but it is most frequently quoted in (km/s)/Mpc, thus giving the speed in km/s of a galaxy 1 megaparsec (3.09×10^{19} km) away. The reciprocal of *H*_{0} is the Hubble time.

In cosmology, a **Hubble volume** or **Hubble sphere** is a spherical region of the observable universe surrounding an observer beyond which objects recede from that observer at a rate greater than the speed of light due to the expansion of the Universe. The Hubble volume is approximately equal to 10^{31} cubic light years.

The proper radius of a Hubble sphere (known as the Hubble radius or the Hubble length) is , where is the speed of light and is the Hubble constant. The surface of a Hubble sphere is called the *microphysical horizon*, the *Hubble surface*, or the *Hubble limit*.

More generally, the term "Hubble volume" can be applied to any region of space with a volume of order . However, the term is also frequently (but mistakenly) used as a synonym for the observable universe; the latter is larger than the Hubble volume.

Hyperion proto-superclusterThe Hyperion proto-supercluster is the largest and earliest known proto-supercluster, 5,000 times the mass of the Milky Way and seen at 20% of the current age of the universe. It was discovered in 2018 by analysing the redshifts of 10,000 objects observed with the Very Large Telescope in Chile.

Isotopes of bariumNaturally occurring barium (56Ba) is a mix of six stable isotopes and one very long-lived radioactive primordial isotope, barium-130, recently identified as being unstable by geochemical means (from analysis of the presence of its daughter xenon-130 in rocks). This nuclide decays by double-electron capture (absorbing two electrons and emitting two neutrinos); with a half-life of (0.5–2.7)×1021 years (about 1011 times the age of the universe).

There are a total of thirty-three known radioisotopes in addition to 130Ba, but most of these are highly radioactive with half-lives in the several millisecond to several minute range. The only notable exceptions are 133Ba, which has a half-life of 10.51 years, 131Ba (11.5 days), and 137mBa (2.55 minutes), which is the decay product of 137Cs (30.17 years, and a common fission product).

Barium-114 is predicted to undergo cluster decay, emitting a nucleus of stable 12C to produce 102Sn. However this decay is not yet observed; the upper limit on the branching ratio of such decay is 0.0034%.

Isotopes of germaniumGermanium (32Ge) has five naturally occurring isotopes, 70Ge, 72Ge, 73Ge, 74Ge, and 76Ge. Of these, 76Ge is very slightly radioactive, decaying by double beta decay with a half-life of 1.78 × 1021 years (130 billion times the age of the universe).

Stable 74Ge is the most common isotope, having a natural abundance of approximately 36%. 76Ge is the least common with a natural abundance of approximately 7%. When bombarded with alpha particles, the isotopes 72Ge and 76Ge will generate stable 75As and 77Se, releasing high energy electrons in the process.At least 27 radioisotopes have also been synthesized ranging in atomic mass from 58 to 89. The most stable of these is 68Ge, decaying by electron capture with a half-life of 270.95 d. It decays to the medically useful positron-emitting isotope 68Ga. (See gallium-68 generator for notes on the source of this isotope, and its medical use). The least stable known germanium isotope is 60Ge with a half-life of 30 ms.

While most of germanium's radioisotopes decay by beta decay, 61Ge and 64Ge decay by β+ delayed proton emission. 84Ge through 87Ge also have minor β− delayed neutron emission decay paths.

Kepler-444Kepler-444 (or KOI-3158, KIC 6278762, 2MASS J19190052+4138043, BD+41 3306) is a star, estimated to be 11.2 billion years old (more than 80% of the age of the universe), approximately 116 light-years (36 pc) away from Earth in the constellation Lyra. On 27 January 2015, the Kepler spacecraft is reported to have confirmed the detection of five sub-Earth-sized rocky exoplanets orbiting the star. According to NASA, no life as we know it could exist on these hot exoplanets, due to their close orbital distances to the host star.

Particle horizonThe particle horizon (also called the cosmological horizon, the comoving horizon (in Dodelson's text), or the cosmic light horizon) is the maximum distance from which particles could have traveled to the observer in the age of the universe. Much like the concept of a terrestrial horizon, it represents the boundary between the observable and the unobservable regions of the universe, so its distance at the present epoch defines the size of the observable universe. Due to the expansion of the universe it is not simply the age of the universe times the speed of light (approximately 13.8 billion light-years), but rather the speed of light times the conformal time. The existence, properties, and significance of a cosmological horizon depend on the particular cosmological model.

Primordial nuclideIn geochemistry, geophysics and geonuclear physics, primordial nuclides, also known as primordial isotopes, are nuclides found on Earth that have existed in their current form since before Earth was formed. Primordial nuclides were present in the interstellar medium from which the solar system was formed, and were formed in the Big Bang, by nucleosynthesis in stars and supernovae followed by mass ejection, by cosmic ray spallation, and potentially from other processes. They are the stable nuclides plus the long-lived fraction of radionuclides surviving in the primordial solar nebula through planet accretion until the present. Only 286 such nuclides are known.

Red dwarfA red dwarf (or M dwarf) is a small and cool star on the main sequence, of M spectral type. Red dwarfs range in mass from about 0.075 to about 0.50 solar mass and have a surface temperature of less than 4,000 K. Sometimes K-type main-sequence stars, with masses between 0.50-0.8 solar mass, are also included.

Red dwarfs are by far the most common type of star in the Milky Way, at least in the neighborhood of the Sun, but because of their low luminosity, individual red dwarfs cannot be easily observed. From Earth, not one is visible to the naked eye. Proxima Centauri, the nearest star to the Sun, is a red dwarf (Type M5, apparent magnitude 11.05), as are fifty of the sixty nearest stars.

According to some estimates, red dwarfs make up three-quarters of the stars in the Milky Way.Stellar models indicate that red dwarfs less than 0.35 M☉ are fully convective. Hence the helium produced by the thermonuclear fusion of hydrogen is constantly remixed throughout the star, avoiding helium buildup at the core, thereby prolonging the period of fusion. Red dwarfs therefore develop very slowly, maintaining a constant luminosity and spectral type for trillions of years, until their fuel is depleted. Because of the comparatively short age of the universe, no red dwarfs exist at advanced stages of evolution.

Steady state modelIn cosmology, the steady state model is an alternative to the Big Bang theory of the evolution of the universe. In the steady state model, the density of matter in the expanding universe remains unchanged due to a continuous creation of matter, thus adhering to the perfect cosmological principle, a principle that asserts that the observable universe is basically the same at any time as well as at any place.

While the steady state model enjoyed some popularity in the mid-20th century (though less popularity than the Big Bang theory), it is now rejected by the vast majority of cosmologists, astrophysicists and astronomers, as the observational evidence points to a hot Big Bang cosmology with a finite age of the universe, which the steady state model does not predict.

Wilkinson Microwave Anisotropy Probe

The **Wilkinson Microwave Anisotropy Probe** (**WMAP**), originally known as the **Microwave Anisotropy Probe** (**MAP**), was a spacecraft operating from 2001 to 2010 which measured temperature differences across the sky in the cosmic microwave background (CMB) – the radiant heat remaining from the Big Bang. Headed by Professor Charles L. Bennett of Johns Hopkins University, the mission was developed in a joint partnership between the NASA Goddard Space Flight Center and Princeton University. The WMAP spacecraft was launched on June 30, 2001 from Florida. The WMAP mission succeeded the COBE space mission and was the second medium-class (MIDEX) spacecraft in the NASA Explorers program. In 2003, MAP was renamed WMAP in honor of cosmologist David Todd Wilkinson (1935–2002), who had been a member of the mission's science team. After nine years of operations, WMAP was switched off in 2010, following the launch of the more advanced Planck spacecraft by ESA in 2009.

WMAP's measurements played a key role in establishing the current Standard Model of Cosmology: the Lambda-CDM model. The WMAP data are very well fit by a universe that is dominated by dark energy in the form of a cosmological constant. Other cosmological data are also consistent, and together tightly constrain the Model. In the Lambda-CDM model of the universe, the age of the universe is ±0.059 billion years. The WMAP mission's determination of the age of the universe is to better than 1% precision. The current expansion rate of the universe is (see Hubble constant) 13.772±0.80 km·s^{−1}·Mpc^{−1}. The content of the universe currently consists of 69.32±0.093% ordinary baryonic matter; 4.628%+0.88%

−0.87% cold dark matter (CDM) that neither emits nor absorbs light; and 24.02%+0.95%

−0.96% of dark energy in the form of a cosmological constant that accelerates the expansion of the universe. Less than 1% of the current content of the universe is in neutrinos, but WMAP's measurements have found, for the first time in 2008, that the data prefer the existence of a cosmic neutrino background with an effective number of neutrino species of 71.35%±0.35. The contents point to a Euclidean flat geometry, with curvature ( 3.26) of +0.0039

−0.0038. The WMAP measurements also support the cosmic inflation paradigm in several ways, including the flatness measurement.
−0.0027

The mission has won various awards: according to *Science* magazine, the WMAP was the *Breakthrough of the Year for 2003*. This mission's results papers were first and second in the "Super Hot Papers in Science Since 2003" list. Of the all-time most referenced papers in physics and astronomy in the INSPIRE-HEP database, only three have been published since 2000, and all three are WMAP publications. Bennett, Lyman A. Page, Jr., and David N. Spergel, the latter both of Princeton University, shared the 2010 Shaw Prize in astronomy for their work on WMAP. Bennett and the WMAP science team were awarded the 2012 Gruber Prize in cosmology. The 2018 Breakthrough Prize in Fundamental Physics was awarded to Bennett, Gary Hinshaw, Norman Jarosik, Page, Spergel and the WMAP science team.

As of October 2010, the WMAP spacecraft is derelict in a heliocentric graveyard orbit after 9 years of operations. All WMAP data are released to the public and have been subject to careful scrutiny. The final official data release was the nine-year release in 2012.

Some aspects of the data are statistically unusual for the Standard Model of Cosmology. For example, the largest angular-scale measurement, the quadrupole moment, is somewhat smaller than the Model would predict, but this discrepancy is not highly significant. A large cold spot and other features of the data are more statistically significant, and research continues into these.

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