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Today's Standard Model of Cosmology is called
LCDM (cold dark matter with a cosmological constant). LCDM tells us that the universe contains about 5% normal matter, 25% cold dark matter, and 70% dark energy and that it started from the Big Bang about 13.6 billion years ago and has been expanding ever since.Some recent observations and analysis, however, suggest that the
LCDM model is in trouble. The Hubble constant H0, the expansion rate of the universe, is taken in LCDM to be a single number, but it has been found to have distinctly different values (by ~10%) when deduced from the cosmic microwave background (H0=67 km/s/mpc) and from red shift vs. distance of observed stars and galaxies (H0=74 km/s/mpc). The universe appears to have expanded more slowly just after the Big Bang than it is expanding now. (See AV-205 in the 03-04-2020 Analog.)Further, in the past year the James Webb Space Telescope has been observing ancient large bright galaxies that appear to have formed much too soon (~500 million years) after the Big Bang. They appear to have the same structure as later galaxies like ours but to have a smaller angular size. There are even a few ancient stars that seem to be significantly older than the
LCDM-derived age of the universe (13.8 Gyr). Among these is HD 140283, which has an estimated age of 14.45±0.8 Gyr. These and many other recent observations suggest that something may be seriously wrong with LCDM, that cosmological effects are missing, and that we need a new cosmology model. There are new ideas for cures to this problem, and some of them cast doubts on the existence of dark matter.So let's begin by considering why cosmologists introduced the concept of dark matter in the first place. The notion that there could be considerable, even dominant, amounts of non-luminous mass in the universe has been kicked around for a long time by some of the pioneers of astronomy, including by Lord Kelvin (1894), Poincare (1904), Jacobus Kapteyn (1922), and Jan Oort (1932).
However, the first real evidence of dark matter came from CalTech's Fritz Zwicky in 1933, when he noticed that the outer stars of the Coma Cluster seemed to be rotating too fast around the center. He applied the virial theorem and concluded that the cluster must have about 400 times more mass than was visually observable. In 1939, Horace W. Babcock reported that the rotation curve (star's tangential velocity vs. distance from center) of the Andromeda Galaxy indicated that the mass-to-luminosity ratio increased with distance from the galactic center, reaching about 50 times more mass than the observed light suggested.
These anomalous rotation-curve observations were widely confirmed by subsequent work, particularly by Vera Rubin in the 1960s and 70s, but remained an astrophysical curiosity for decades. More recently, analysis of gravitational lensing, the temperature distribution of hot gas in galaxies, and angle-dependent variations in the cosmic microwave background have all provided additional evidence for the presence of a large quantity (~25% of all mass-energy) of dark matter in the universe. Consequently, dark matter plays a central role in the
LCDM standard model of cosmology.Today, the accumulated evidence in support of the presence of dark matter in the universe forms an impressive “bullwark” of seemingly unrelated physical effects and observations, all pointing in the same direction. These include: galactic rotation curves, velocity dispersions, x-rays from hot gas, gravitational lensing, the cosmic microwave background, galactic structure formation after the Big Bang, center-of-mass displacement of the Bullet Cluster, baryon acoustic oscillations, large-scale redshift-space distortions, and shifts in the Lyman-alpha forest.
This body of evidence has triggered great effort among experimental physicists to find and identify particles that form this excess invisible mass. The list of possible dark-matter particles includes axions (see
AV-191 in the 11-12-2019 Analog), massive neutrinos, weakly interacting massive particles or WIMPs (see AV-92 in the 09-1998 Analog), super-symmetry particles, light Higgs bosons, primordial black holes (see AV-230 in the 05-06-2023 Analog), and massive cosmic halo objects or MACHOs (see AV-65 in the 11-1994 Analog). At this writing, some of these candidates have been eliminated but none has been identified as the source of dark matter. Some astrophysicists are beginning to doubt that dark matter actually exists.Any new idea for fixing problems with
LCDM must be able to accommodate the large body of evidence that supports the existence of dark matter. That is not easy. Here I want to review two new theoretical initiatives, both of which involve explaining at least some of the observations listed above without requiring dark matter.The first of these is the work of Jonathan Oppenheim and his coworkers at University College London. They have built on the 1987 work of Mordechai Milgrom. Milgrom's idea, called MOND (modification of Newtonian dynamics), demonstrated that if the smallest possible values of Newtonian acceleration aN becomes (aNa0)1/2 when aN is less than a0, with a0 ~ 10-10 m/s2, then the non-falloff of the galactic rotation curves is reproduced. This eliminates the need for dark matter. Milgrom's acceleration-limiting process, however, was not explained by any known physics,
Oppenheim had previously introduced an alternative to quantum gravity, joining gravitation with quantum mechanics by using Feynman path integrals. His group has recently shown that this formalism leads to a gravitational entropic force, driven by a stochastic cosmological constant, that limits the minimum acceleration, as MOND had suggested, and explains galactic rotation curves without needing dark matter.
Oppenheim's theory is testable with tabletop experiments, if these can reach the needed precision. However, it is not clear, at this writing, if Oppenheim's minimum-acceleration fluctuations can also explain the many other observations listed above that support the existence of dark matter.
The other theoretical initiative is the work of Rajendra P. Gupta of the University of Ottawa. It is given the label CCC+TL cosmology, which stands for the covariation of coupling constants with the tired light hypothesis. Essentially, Gupta allows the physical constants c (speed of light), h (Planck's constant), G (Newton's gravitational constant, and k (Boltzman's constant) to vary together over time in a correlated way. Using this approach, he is able to explain many of the recent JWST observational surprises and to make the universe older (26.7 Gyr) by almost a factor of two.
The idea of changing physical constants is not new. In 1937 Dirac, using his large number hypothesis, predicted time-dependent variation of G. Others have considered the effects of such variations on astrophysical observables, e.g., neutron-star masses and ages, cosmic microwave background temperature anisotropies, and Big Bang nucleosynthesis, and have set constraints on possible variations of G that are well below Dirac's prediction.
In 1907, when Einstein developed special relativity, he considered the possibility of a variation of the vacuum value of c. Since that time, many others have entertained theories that include c variations. However, there is no evidence of any such variation of c.
Gupta's idea is to vary the four physical constants together, so that they all depend on the same changing function f(a), with: G=G0 f(a), c=c0 f(a)3, h=h0 f(a)2,
and k=k0 f(a)2, with f(a) varying with time to fit observational data. Here a is the time-varying scale factor of the expanding universe, and the constants with 0 subscript are today's values of these constants. Using this approach, combined with the “tired light” assumption, he is able to account for the small angular size of early galaxies, the primordial-lithium problem, the faint-young-sun problem, Type Ia supernova luminosity, and the impossibly early galaxy formation problem. He has also been able to show that CCC+TL is compatible with observations related to core-collapse during supernova explosions, orbital timing tests, and gravitational lensing.Gupta asserts that the CCC+TL model is equivalent to the
LCDM model with L allowed a time-variation. It eliminates the need for dark matter. However, it has not (yet) been able to explain several astronomical observational phenomena attributed to dark matter, e.g., galactic rotation curves or the masses of galactic clusters.So, has dark matter gone the way of ether and phlogiston as constituents of the universe? Not yet. The observational and analytic evidence is support of the presence of dark matter in the universe is quite large, and neither Oppenheim nor Gupta are able to account for all of it.
One issue that is particularly worrisome (at least to me) is the problem of heavy element nucleosynthesis in stars. After decades of progress in nuclear astrophysics, we now understand that all the elements between lithium and nickel were slowly created in the pressure cooker of evolving stars, as they burned hydrogen to helium, helium to carbon, and carbon to heavier elements up to nickel. Then, when these large stars went supernova, the elements heavier that nickel were created by rapid absorption of neutrons and decays to stability.
As first pointed out by Fred Hoyle, this process has a critical bottleneck: the nucleus 8Be, the stepping stone between 4He and 12C, is unstable to alpha-particle decay, with a half life of only 8 × 10-17 seconds. In the hot pressure-cooker of a stellar core, it is produced by the collision of two 4He nuclei and lives only long enough to occasionally collide with a third 4He to form an excited state of 12C, which then decays with two gamma rays and becomes a stable nucleus. However, as Hoyle pointed out, for this process to occur, the 12C nucleus must have a then-unknown resonant state, a Jπ
=0+ 2nd excited state with an energy of 7.654 million electron volts above the ground state. Hoyle predicted in 1954 that this state existed, and shortly thereafter it was discovered by nuclear experimentalists.The exact excitation energies of nuclear excited states depend in a complicated way on the strong and electromagnetic forces and on the quantum tendency for nuclei to form angular-momentum-based “closed shells” for stability. Any tinkering with minimum acceleration limits or variation of fundamental constants is likely to alter the excitation energies of these states.
If the 7.654 MeV state of 12C moved only slightly, the chain of heavy element nucleosynthesis in stars would be broken, elements heavier that helium would not be efficiently synthesized, and we, with bodies and brains made of these elements, would not be here to worry about it.
Gupta happened to be in Seattle recently, and I had lunch with him and asked him whether CCC+TL variations would move the Hoyle level of 12C. His answer was that it would not, but I am concerned about his answer. He said that the level position depends on hc/kT, and this ratio does not change as CCC is applied. My problems with this answer are that (a) nuclear energy level positions are not temperature-dependent, and there is no obvious reason why Boltzman's constant k is relevant, and (b) using the suggested CCC procedure, the suggested ratio would not be constant but should change as f(a)3.
In conclusion, on the horizon there are reasons why the presence of dark matter in the universe is in doubt. However, the alternatives to LCDM, while solving some problems, cannot at present account for all the observations that support the presence of dark matter. The new problem-solving alternative to LCDM remains to be found.
John
G. Cramer's 2016 nonfiction book describing his transactional interpretation of
quantum mechanics, The Quantum Handshake - Entanglement, Nonlocality, and Transactions,
(Springer, January-2016) is available online as a hardcover or eBook at:
http://www.springer.com/gp/book/9783319246406 or
https://www.amazon.com/dp/3319246402
.
SF Novels:
John's 1st hard SF novel Twistor
is available online at:
Alternate
View Columns Online: Electronic reprints of 227 or more of "The
References:
J. Oppenheim and A. Russo, "Anomalous contribution to galactic rotation curves due to stochastic spacetime," arXiv:2402.19459v2 [gr-qc].
Rajendra P. Gupta, "JWST early Universe observations and LCDM cosmology," MNRAS 524, 3385–3395 (2023); arXiv:2309.13100v1 [gr-qc].
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