Dark energy
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In physical cosmology and astronomy, dark energy is an unknown 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,[1] dark energy is the dominant component of the universe, contributing 68% of the total energy in 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.[2][3][4][5] Dark energy's density is very low: 6×10−10 J/m3 (≈7×10−30 g/cm3), much less than the density of ordinary matter or dark matter within galaxies. However, it dominates the universe's mass–energy content because it is uniform across space.[6][7][8]
The first observational evidence for dark energy's existence came from measurements of
The exact nature of dark energy remains a mystery, and explanations abound. The main candidates are a cosmological constant[11][12] (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 the Theories of Dark Energy section).
History of discovery and previous speculation
Einstein's cosmological constant
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 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.[13] 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 the interstellar space'.[14][15]
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, 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.[16] These sorts of disturbances are inevitable, due to the uneven distribution of matter throughout 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.[17]
Inflationary dark energy
Nearly all inflation models predict that the total (matter+energy) density of the universe should be very close to the
The term "dark energy", echoing Fritz Zwicky's "dark matter" from the 1930s, was coined by Michael S. Turner in 1998.[20]
Change in expansion over time
High-precision measurements of 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 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 the
Nature
The nature of dark energy is more hypothetical than that of dark matter, and many things about it remain in the realm of speculation.
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.[23] The mass–energy equivalence postulated by general relativity implies that 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; the problem remains unresolved.
Independently of its actual nature, dark energy would need to have a strong negative pressure to explain the observed
Technical definition
In standard cosmology, there are three components of the universe: matter, radiation, and dark energy. Matter is anything whose energy density scales with the inverse cube of the scale factor, i.e., ρ ∝ a−3, while radiation is anything which scales to the inverse fourth power of the scale factor (ρ ∝ a−4). 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 (23). For radiation, the decrease in energy density is greater, because an increase in spatial distance also causes a redshift.[24]
The final component is dark energy: it is an intrinsic property of space and has a constant energy density, regardless of the dimensions of the volume under consideration (ρ ∝ a0). Thus, unlike ordinary matter, it is not diluted by the expansion of space.
Evidence of existence
The evidence for dark energy is indirect but comes from three independent sources:
- Distance measurements and their relation to redshift, which suggest the universe has expanded more in the latter half of its life.[25]
- The theoretical need for a type of additional energy that is not matter or dark matter to form the observationally flat universe(absence of any detectable global curvature).
- Measures of large-scale wave patterns of mass density in the universe.
Supernovae
In 1998, the
Since then, these observations have been corroborated by several independent sources. Measurements of the
Supernovae are useful for cosmology because they are excellent
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.[31]
Large-scale structure
The theory of large-scale structure, which governs 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.
Cosmic microwave background
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 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 the remaining 70%.[29] 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.[4] Work done in 2013 based on the
Late-time integrated Sachs–Wolfe effect
Accelerated cosmic expansion causes
Observational Hubble constant data
A new approach to test evidence of dark energy through observational
The Hubble constant, H(z), 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".[43] 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 a direct estimate of the Hubble parameter
The reliance on a differential quantity, Δz/Δt, 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 Δz/Δt measures it directly. For these reasons, this method has been widely used to examine the accelerated cosmic expansion and study properties of dark energy.[citation needed]
Theories 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.
Cosmological constant
The simplest explanation for dark energy 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 = mc2, Einstein's theory of general relativity predicts that this energy will have a gravitational effect. It is sometimes called a vacuum energy because it is the energy density of empty space – a vacuum.
A major outstanding
Some supersymmetric theories require a cosmological constant that is exactly zero.[46] Also, it is unknown if there is a metastable vacuum state in string theory with a positive cosmological constant,[47] and it has been conjectured by Ulf Danielsson et al. that no such state exists.[48] This conjecture would not rule out other models of dark energy, such as quintessence, that could be compatible with string theory.[47]
Quintessence
In
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
The coincidence problem asks why the
In 2004, when scientists fit the evolution of dark energy with the cosmological data, they found that 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.[52]
Some special cases of quintessence are
A group of researchers argued in 2021 that observations of the
Interacting dark energy
This class of theories attempts to come up with an all-encompassing theory of both dark matter and dark energy as 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,[55] or postulate that cold dark matter decays into dark energy.[56] 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 the modified dynamics stems to what have been assigned to the presence of dark energy and dark matter.[57] 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.[58] It 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.[59][60]
Variable dark energy models
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 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).[61][62] Some other common models are (Barboza & Alcaniz. 2008),[63] (Jassal et al. 2005),[64] (Wetterich. 2004),[65] and (Oztas et al. 2018).[66][67]
Possibly decreasing levels
Researchers using the Dark Energy Spectroscopic Instrument (DESI) to make the largest 3-D map of the universe at this point (2024),[68] 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.[69]
Observational skepticism
Some alternatives to dark energy, such as
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[82] was quickly countered by another paper claiming errors in the original paper.[83] Another study questioning the essential assumption that the luminosity of Type Ia supernovae does not vary with stellar population age[84][85] was also swiftly rebutted by other cosmologists.[86]
As a general relativistic effect due to black holes
This theory was formulated by
Other mechanism driving acceleration
Modified gravity
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 the need for dark energy. There are many such theories, and research is ongoing.[91][92] 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.[93][94][95]
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 the competing theories successfully explain observations to the same level of precision as standard dark energy.[96]
Non-linearities of General Relativity equations
The
Implications for the fate of the universe
Cosmologists estimate that the acceleration began roughly 5 billion years ago.[98][a] 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 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, 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.[99] 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 the speed of light for any massive object (see Uses of the proper distance for a discussion of the subtleties of defining any notion of relative velocity in cosmology). Because the Hubble parameter is decreasing with time, there can actually be cases where a galaxy that is receding from us faster than light does manage to emit a signal which reaches us eventually.[100][101]
However, because of the accelerating expansion, it is projected that most galaxies will eventually cross a type of cosmological event horizon where any light they emit past that point will never be able to reach us at any time in the infinite future[102] because the light never reaches a point where its "peculiar velocity" toward us exceeds the expansion velocity away from us (these two notions of velocity are also discussed in Uses of the proper distance). Assuming the dark energy is constant (a cosmological constant), the current distance to this cosmological event horizon is about 16 billion light years, meaning that a signal from an event happening at present would eventually be able to reach us in the future if the event were less than 16 billion light years away, but the signal would never reach us if the event were more than 16 billion light years away.[101]
As galaxies approach the point of crossing this cosmological event horizon, the light from them will become more and more redshifted, to the point where the wavelength becomes too large to detect in practice and the galaxies appear to vanish completely[103][104] (see Future of an expanding universe). Planet Earth, the Milky Way, and the Local Group of galaxies of which the Milky Way is a part, would all remain virtually undisturbed as the rest of the universe recedes and disappears from view. In this scenario, the Local Group would ultimately suffer heat death, just as was hypothesized for the flat, matter-dominated universe before measurements of cosmic acceleration.[citation needed]
There are other, more speculative ideas about the future of the universe. The
In philosophy of science
The astrophysicist David Merritt identifies dark energy as an example of an "auxiliary hypothesis", an ad hoc postulate that is added to a theory in response to observations that falsify it. He argues that the dark energy hypothesis is a conventionalist hypothesis, that is, a hypothesis that adds no empirical content and hence is unfalsifiable in the sense defined by Karl Popper.[108] However, his opinion does not seem to be consensus[by whom?] and is at odds with the history of cosmology.[why?][109]
See also
Notes
- ^ Taken from Frieman, Turner, & Huterer (2008):[98]: 6, 44
The Universe has gone through three distinct eras:
- Radiation-dominated, z ≳ 3000 ;
- Matter-dominated, 3000 ≳ z ≳ 0.5 ; and
- Dark-energy-dominated, 0.5 ≳ z .
The evolution of the scale factor is controlled by the dominant energy form:
(for constant w ). During the radiation-dominated era,
during the matter-dominated era,
and for the dark energy-dominated era, assuming w ≃ −1 asymptotically
- [98]: 6
Taken together, all the current data provide strong evidence for the existence of dark energy; they constrain the fraction of critical density contributed by dark energy, 0.76 ± 0.02 , and the equation-of-state parameter:
- w ≈ −1 ± 0.1 [stat.] ± 0.1 [sys.] ,
assuming that w is constant. This implies that the Universe began accelerating at redshift z ~ 0.4 and age t ~ 10
Ga . These results are robust – data from any one method can be removed without compromising the constraints – and they are not substantially weakened by dropping the assumption of spatial flatness.[98]: 44
References
- S2CID 119249858.
- S2CID 218716838.
- S2CID 218716838. Archived from the originalon 23 March 2013.
- ^ a b "First Planck results: the Universe is still weird and interesting". 21 March 2013. Archived from the original on 2 May 2019. Retrieved 14 June 2017.
- ^ Sean Carroll, Ph.D., Caltech, 2007, The Teaching Company, Dark Matter, Dark Energy: The Dark Side of the Universe, Guidebook Part 2. p. 46. Retrieved 7 October 2013, "...dark energy: A smooth, persistent component of invisible energy, thought to make up about 70 percent of the current energy density of the universe. Dark energy is known to be smooth because it doesn't accumulate preferentially in galaxies and clusters..."
- S2CID 14178620.
- ^ "Dark Energy". Hyperphysics. Archived from the original on 27 May 2013. Retrieved 4 January 2014.
- ^ Ferris, Timothy (January 2015). "Dark Matter(Dark Energy)". National Geographic Magazine. Archived from the original on 10 June 2015. Retrieved 10 June 2015.
- ^ Overbye, Dennis (20 February 2017). "Cosmos Controversy: The Universe Is Expanding, but How Fast?". The New York Times. Archived from the original on 4 April 2019. Retrieved 21 February 2017.
- from the original on 7 January 2024.
- ^ Cookson, Clive (3 June 2011). "Moon findings muddy the water". Financial Times. Archived from the original on 22 November 2016. Retrieved 21 November 2016.
- ^ PMID 28179856. Archived from the originalon 13 October 2006. Retrieved 28 September 2006.
- arXiv:1211.6338 [physics.hist-ph].
- ^ "Volume 7: The Berlin Years: Writings, 1918-1921 (English translation supplement) page 31". einsteinpapers.press.princeton.edu. Retrieved 18 September 2023.
- ^ O'Raifeartaigh, C.; O'Keeffe, M.; Nahm, W.; Mitton, S. (2017). 'Einstein's 1917 Static Model of the Universe: A Centennial Review'. Eur. Phys. J. (H) 42: 431–474.
- ^ "Dark Energy, Dark Matter". Science Mission Directorate. Archived from the original on 5 November 2020. Retrieved 17 September 2022.
- ^ Gamow, George (1970) My World Line: An Informal Autobiography. p. 44: "Much later, when I was discussing cosmological problems with Einstein, he remarked that the introduction of the cosmological term was the biggest blunder he ever made in his life." – Here the "cosmological term" refers to the cosmological constant in the equations of general relativity, whose value Einstein initially picked to ensure that his model of the universe would neither expand nor contract; if he had not done this he might have theoretically predicted the universal expansion that was first observed by Edwin Hubble.
- ^ S2CID 15640044.
- ^ S2CID 118910636.
- S2CID 119427069.
- S2CID 119344498.
- ^ Overbye, Dennis (22 July 2003). "Astronomers Report Evidence of 'Dark Energy' Splitting the Universe". The New York Times. Archived from the original on 26 June 2015. Retrieved 5 August 2015.
- from the original on 30 November 2010. Retrieved 29 October 2022.
- ^ Baumann, Daniel. "Cosmology: Part III Mathematical Tripos, Cambridge University" (PDF). p. 21−22. Archived from the original (PDF) on 2 February 2017. Retrieved 31 January 2017.
- S2CID 17562830.
- S2CID 116951785.
- ^ "The Nobel Prize in Physics 2011". Nobel Foundation. Archived from the original on 1 August 2012. Retrieved 4 October 2011.
- ^ The Nobel Prize in Physics 2011 Archived 4 October 2011 at the Wayback Machine. Perlmutter got half the prize, and the other half was shared between Schmidt and Riess.
- ^ (PDF) from the original on 6 April 2020. Retrieved 26 December 2019.
- S2CID 17562830.
- sys), of the total matter density, ΩM, of 0.274+0.016–0.016(stat)+0.013–0.012(sys) with an equation of state parameterw of −0.969+0.059–0.063(stat)+0.063–0.066(sys).
- ^ "New method 'confirms dark energy'". BBC News. 19 May 2011. Archived from the original on 15 June 2018. Retrieved 21 July 2018.
- ^ a b Dark energy is real Archived 25 May 2011 at the Wayback Machine, Swinburne University of Technology, 19 May 2011
- ^ "Content of the Universe – Pie Chart". Wilkinson Microwave Anisotropy Probe. National Aeronautics and Space Administration. Archived from the original on 18 August 2018. Retrieved 9 January 2018.
- ^ "Big Bang's afterglow shows universe is 80 million years older than scientists first thought". The Washington Post. Archived from the original on 22 March 2013. Retrieved 22 March 2013.
- S2CID 119012700.
- S2CID 38383124.
- S2CID 21763795.
- S2CID 8220261.
- S2CID 119125999.
- S2CID 119181595.
- S2CID 62885316.
- S2CID 13215290.
- ^ by Ehsan Sadri Astrophysics MSc, Azad University, Tehran
- ESA. 21 March 2013. Archivedfrom the original on 6 December 2013. Retrieved 21 March 2013.
- ISBN 978-0691025308.
- ^ a b Wolchover, Natalie (9 August 2018). "Dark energy may be incompatible with string theory". Quanta Magazine. Simons Foundation. Archived from the original on 15 November 2020. Retrieved 2 April 2020.
- S2CID 119198922.
- S2CID 14539052.
- PMID 9958635.
- S2CID 40714104.
- S2CID 118866606.
- S2CID 9820570.
- S2CID 234790314.
- ^ See dark fluid.
- arXiv:1610.01272 [astro-ph.CO].
- S2CID 119169726.
- .
- ^ "A new dark matter experiment quashed earlier hints of new particles". Science News. 22 July 2022. Archived from the original on 26 August 2022. Retrieved 3 August 2022.
- S2CID 251040527.
- S2CID 16489484.
- S2CID 16219710.
- S2CID 118306372.
- S2CID 9144993.
- S2CID 119354763.
- .
- .
- ISBN 3-540-43769-X,
- ISBN 3-540-43769-X, retrieved 13 April 2024
- S2CID 1152275.
- S2CID 118801032.
- S2CID 14226736.
- PMID 19363920.
- S2CID 53709630.
- ^ Gray, Stuart (8 December 2009). "Dark questions remain over dark energy". ABC Science Australia. Archived from the original on 15 January 2013. Retrieved 27 January 2013.
- ^ Merali, Zeeya (March 2012). "Is Einstein's Greatest Work All Wrong – Because He Didn't Go Far Enough?". Discover magazine. Archived from the original on 28 January 2013. Retrieved 27 January 2013.
- ^ Wolchover, Natalie (27 September 2011) 'Accelerating universe' could be just an illusion Archived 24 September 2020 at the Wayback Machine, NBC News
- S2CID 119179171.
- PMID 27767125.
- WP:NEWSBLOG). Archivedfrom the original on 26 July 2017. Retrieved 10 August 2017.
- S2CID 118935116.
- S2CID 208175643.
- S2CID 208637339.
- ^ Yonsei University (6 January 2020). "New evidence shows that the key assumption made in the discovery of dark energy is in error". Phys.org. Archived from the original on 13 January 2020. Retrieved 6 January 2020.
- S2CID 209202868.
- ^ Gohd, Chelsea (9 January 2020). "Has Dark Energy Been Debunked? Probably Not". Space.com. Archived from the original on 2 March 2020. Retrieved 14 February 2020.
- ^ "Wait... Did We Finally Find the Source of Dark Energy?!". MSN. Retrieved 4 April 2023.
- ^ Siegel, Ethan (17 February 2023). "Ask Ethan: Can black holes really cause dark energy?". Starts with a Bang.
- ^ Rodriguez, Carl L. "No, black holes are not the source of dark energy". Retrieved 11 September 2023.
- S2CID 259165172.
- S2CID 119256879. for a recent review
- S2CID 118468001.
- S2CID 118486016.
- ^ "Quest to settle riddle over Einstein's theory may soon be over". phys.org. 10 February 2017. Archived from the original on 28 October 2017. Retrieved 29 October 2017.
- ^ "Theoretical battle: Dark energy vs. modified gravity". Ars Technica. 25 February 2017. Archived from the original on 28 October 2017. Retrieved 27 October 2017.
- ^ Siegel, Ethan (2018). "What Astronomers Wish Everyone Knew About Dark Matter And Dark Energy". Forbes (Starts With A Bang blog). Archived from the original on 11 April 2018. Retrieved 11 April 2018.
- .
- ^ S2CID 15117520.
- ^ Krauss, Lawrence M.; Scherrer, Robert J. (March 2008). "The End of Cosmology?". Scientific American. 82. Archived from the original on 19 March 2011. Retrieved 6 January 2011.
- ^ Is the universe expanding faster than the speed of light? Archived 23 November 2003 at the Wayback Machine (see the last two paragraphs)
- ^ a b Lineweaver, Charles; Davis, Tamara M. (2005). "Misconceptions about the Big Bang" (PDF). Scientific American. Archived from the original (PDF) on 19 July 2011. Retrieved 6 November 2008.
- S2CID 1791226.
- S2CID 123442313.
- ^ Using Tiny Particles To Answer Giant Questions Archived 6 May 2018 at the Wayback Machine. Science Friday, 3 April 2009. According to the transcript Archived 6 May 2018 at the Wayback Machine, Brian Greene makes the comment "And actually, in the far future, everything we now see, except for our local galaxy and a region of galaxies will have disappeared. The entire universe will disappear before our very eyes, and it's one of my arguments for actually funding cosmology. We've got to do it while we have a chance."
- ^ How the Universe Works 3. Vol. End of the Universe. Discovery Channel. 2014.
- ^ "'Cyclic universe' can explain cosmological constant". New Scientist. Retrieved 18 September 2023.
- S2CID 1346107.
- S2CID 119401938.
- Bibcode:2020Obs...140..225H.
External links
- Euclid ESA Satellite, a mission to map the geometry of the dark universe
- "Surveying the dark side" by Roberto Trotta and Richard Bower, Astron.Geophys.