Major issues with ΛCDM

  • “Expansion of space” has no empirical test beyond the realm of astronomical observations.
  • The unknown nature of the main ingredients of the model, “dark matter” and “dark energy”.
  • Smallness of Λ (> 0).
  • The unknown physics of “inflation”.
  • Validity of General Relativity assumed beyond experimentally tested scales.
  • Tensions that appear with every precision measurements.
  • High uncertainty on the most fundamental parameter of the model \(H_0\).
  • Small-scale Issues: Core-Cusp, Satellite, etc.

Observations at tension with cosmological models

By Louis Marmet

The Hubble tension: disagreement at 5σ significance

A discrepancy between the value of Hubble parameter \(H_0\) predicted from the cosmological model using measurements of the early Universe, and the value more directly measured from the late Universe arXiv:2001.03624.

The Hubble tension problem

Fine tuning or cosmological constant problem

The observed value of the cosmological constant Λ is 120 values less than its theoretically predicted value.

Coincidence problem

According to the data of the Planck satellite, at present epoch the cosmological constant energy density (68.5%) is comparable (in order of magnitude) with the energy density of matter (31.5%), despite the fact that these quantities have evolved differently).

The parameter \(S_8 = \sigma_8 \sqrt{\Omega_m/0.3}\) tension problem

A discrepancy between the primary CMB temperature anisotropy measurements by the Planck satellite in the strength of matter clustering compared to lower redshift measurements such as the weak gravitational lensing and galaxy clustering. Here \(\sigma_8\) is the clustering amplitude of the matter power spectrum on scales of \(8h^{-1}\) Mpc, \(\Omega_m\) is a fractional matter density parameter.

Concordance values that don’t fit

A QSO Hubble diagram

CMB alignments and the ‘Axis of evil’

  • Multipoles of the CMB are aligned with the solar system.
  • Anomalies in the CMB (alignment quadrupole/octopole, insufficient lens effect in clusters, etc.)

Inconsistent with a random isotropic sky

CMB that appears to originate from intergalactic dust

Intergalactic dust

The dipole anisotropy in the number of radio galaxies

The dipole anisotropy in the number of radio galaxies appears larger than that of the CMB.

Unobserved predictions of ΛCDM

From B. Famaey, S.S. McGaugh, Modified Newtonian Dynamics (MOND): Observational Phenomenology and Relativistic Extensions Living Rev. Relativ. 15, 10 (2012)., and Checking in on Troubles with Dark Matter

The bulk flow challenge

Peculiar velocities of galaxy clusters are predicted to be on the order of 200 km/s in the ΛCDM model: as massive, recently formed objects, they should be nearly at rest with respect to the frame of the cosmic microwave background. Instead, they are observed to have bulk flows of order 1000 km/s.

The high-z clusters challenge

Structure formation is reputed to be one of the greatest strengths of ΛCDM, but the observers’ experience has consistently been to find more structure in place earlier than expected. This goes back at least to the 1987 CfA redshift survey stick man figure, which may seem normal now but surprised the bejeepers out of us at the time. It also includes clusters of galaxies, which appear at higher redshift than they should. At the time, we pointed out XMMU J2235.3-2557 with a mass of \(\sim 4 \times 10^{14} M_\odot\) at \(z = 1.4\) as being very surprising.

More recently we have El Gordo, so this remains a problem.

The Local Void challenge

Peebles has been pointing out for a long time that voids are more empty than they should be, and do not contain the population of galaxies expected in LCDM. They’re too normal, too big, and gee it would help if structure formed faster. In our review, we pointed out that the “Local Void” hosts only 3 galaxies, which is much less than the expected ∼20 for a typical similar void in ΛCDM.

The angular momentum challenge

During galaxy formation, the baryons sink to the centers of their dark matter halos. A persistent idea is that they spin up as they do so (like a figure skater pulling her arms in), ultimately establishing a rotationally supported equilibrium in which the galaxy disk is around ten or twenty times smaller than the dark matter halo that birthed it, depending on the initial spin of the halo. This is a seductively simple picture that still has many adherents despite never having really worked. In live simulations, in which baryonic and dark matter particles interact, there is a net transfer of angular momentum from the baryonic disk to the dark halo. This results in simulated disks being much too small.

The pure disk challenge

Structure forms hierarchically in CDM: small galaxies merge into larger ones. This process is hostile to the existence of dynamically cold, rotating disks, preferring instead to construct dynamically hot, spheroidal galaxies. All the merging destroys disks. Yet spiral galaxies are ubiquitous, and many late type galaxies have no central bulge component at all. At some point it was recognized that the existence of quiescent disks didn’t make a whole lot of sense in LCDM. To form such things, one needs to let gas dissipate and settle into a plane without getting torqued and bombarded by lots of lumps falling onto it from random directions. Indeed, it proved difficult to form large, bulgeless, thin disk galaxies in simulations.

The stability challenge

One of the early indications of the need for spiral galaxies to be embedded in dark matter halos was the stability of disks. Thin, dynamically cold spiral disks are everywhere around us, yet Newton can’t hold them together by himself: simulated spirals self destruct on a short timescale (a few orbits). A dark matter halo precludes this from happening by counterbalancing the self-gravity of the disk. This is a somewhat fine-tuned situation: too little halo, and a disk goes unstable; too much and disk self-gravity is suppressed – and spiral arms and bars along with it.

The missing baryons challenge

The cosmic fraction of baryons – the ratio of normal matter to dark matter – is well known (16 ± 1%). One might reasonably expect individual CDM halos to be in in possession of this universal baryon fraction: the sum of the stars and gas in a galaxy should be 16% of the total, mostly dark mass. However, most objects fall well short of this mark, with the only exception being the most massive clusters of galaxies.

Dark Energy is not seen by the ‘Dark Energy Survey’

  • The analysis results in a 2.4σ tension with the DES Y1 3x2pt results, and in 5.6σ with the Planck CMB analysis; arXiv:2002.11124.

CMBR: The σ8 tension

  • A problem of the ΛCDM model is the tension between the relatively high level of clustering, as quantified by the parameter σ8, found in cosmic microwave background experiments and the smaller one obtained from large-scale observations in the late Universe; The tension between KiDS cosmic shear and the Planck-Legacy CMB measurements remains in this systematically more robust analysis, with S8 differing by 2.3σ;

  • Reionization epoch different from CMBR and QSO observations.

Dark matter is not observed in the laboratory

  • The exotic, weakly interacting particles envisaged as candidates for the dark matter component are still undetected in the laboratory.
  • Discovery of baryonic matter with more sensitive instruments decreases the needed amount of dark matter.  The estimated amount of dark matter in the Coma cluster is now a factor of 4 to 5 larger than baryonic matter.  Due to the discovery of hot gas in rich galaxy clusters it is ∼100 less than when Zwicky first discovered it.
  • The “Train Wreck” Cluster, Abell 520, is not understood in Standard Cosmology;;…668..806M/abstract;

Matter-antimatter asymmetry

There is as of yet no consensus theory to explain the baryon asymmetry problem. There is no observation of antimatter or evidence for CP violation.

The gap between prediction and observation is attributed to an unknown process that produced baryon number non-conservation - more baryons than anti-baryons.  Such processes have never been observed in any experiments on earth.

Any such processes would necessarily imply a finite lifetime for the proton of about 1030 years, as originally predicted by Georgi, Quinn and Weinberg, as a necessary condition for resolving the antimatter problem. As experiments ruled out longer and longer lifetimes, these Grand Unified Theories theories were modified to predict longer lifetimes.  However, to date observations have ruled out a lifetime even 104 times larger excluding all these theories.  There is no experimental evidence of a finite lifetime for the proton.  But the lack of such a finite lifetime would rule out the baryon number non-conservation needed to overcome the 1011-fold gap between Big Bang baryon density predictions and observations.  The inconsistency between prediction and observation still exists.

Missing Baryon problem

The observed amount of baryonic matter does not match theoretical predictions, yielding about 50% of the expected baryonic density.  There is not enough matter to produce large galaxies in a time period not exceeding the age of the universe. “Hot baryons within the virial radius of massive galaxy halos are insufficient to explain the ‘‘missing baryons.’’ApJL 2018.

The ‘missing baryons problem’, McGaugh, “The Halo by Halo Missing Baryon Problem. In: Dark Galaxies and Lost Baryons,” Proceedings of the International Astronomical Union 244, p. 136, 2008.


  • The ‘Lithium problem’; B.D. Fields, “The primordial lithium problem” Annual Review of Nuclear and Particle Science 61, p. 47, 2011. Also:
  • Galaxies with 4He < 24%.
  • Ill-understood deuterium abundances, failure in the predictions of Li, Be, 3He.
  • Higher metallicity and dust content at high redshift than expected.

Redshift anomalies

  • The K-Trumpler effect: an excess of redshift observed on the more luminous stars of a cluster.  “Redshifts of high-luminosity stars - the K effect, the Trumpler effect and mass-loss corrections,” Mon. Not. R. astr. Soc. 258, 800-810, 1992;
  • Spatial correlations between objects at low redshift and objects at high redshift, galaxy-quasar associations, periodicity of redshifts; “Physical association and periodicity in quasar families with SDSS and 2MRS,” Astrophysics and Space Science 363:134, July 2018.
  • Intrinsic and anomalous redshifts; “Fingers of God” in redshift space.
  • Companion galaxies have redshifts averaging 122±34 km/s higher than that of the dominant galaxy in clusters. [Arp, H. Ap.J. 256 (1982): 54-74.]
  • ΛCDM fails the Tolman test: the surface brightness signal does not follow (1 + z)-4; Lubin and Sandage, “The Tolman Surface Brightness Test for the Reality of the Expansion. IV. A Measurement of the Tolman Signal and the Luminosity Evolution of Early-Type Galaxies,” The Astronomical Journal 122, no. 3, p. 1084, Sep. 2001,, arXiv:astro-ph/0106566
  • There is no evidence of time dilation in the duration of gamma-ray bursts arXiv:0901.4169.
  • Intergalactic medium temperature independent of redshift arXiv:1009.0953.

Surface-Brightness measurements are explained with galactic size evolution

  • Galaxies must increase in size at a rate larger than allowed by galaxy mergers.
  • The extreme evolution of galaxy sizes is poorly understood.

Failure to detect the Transverse Proximity Effect with a foreground quasar

Contrived explanations are necessary to explain the absence of a transverse proximity effect in hundreds of observations of projected quasar pairs; arXiv:0809.2277 and arXiv:1809.04614.  Observations are explained if QSOs have had their current UV luminosities for less than approximately a million years (coincidentally, we always observe just after an increase of the UV luminosity), or an anisotropic emission from a QSO located within an obscuring torus of gas and dust (always conveniently producing a very narrow emission angle).

Large-Scale structures are too large

  • The universe is inhomogeneous at a scale incompatible with the smooth and homogeneous beginning implied by the Big Bang theory.
  • Flows of large-scale structure matter with excessive velocity.
  • Inhomogeneities at scales > 200 Mpc.

The faint blue galaxy problem

Deeper observations show a great excess of faint galaxies.  Most F.B.G.s appear between red-shift 0.5 and 2. It is believed that they disappear as separate objects by merger with other galaxies

An Accreting Supermassive Black Hole 570 Myr after the Big Bang

This presently highest-redshift AGN requires “super-Eddington accretion” from stellar seeds.


  • The missing satellites problem; B. Moore, et al., “Dark matter substructure within galactic halos” The Astrophysical Journal Letters 524, pp. L19-L22, 1999.
  • Satellite planes: unanticipated correlations in phase space.
  • Excessive cluster densities.
  • The emptiness of voids.
  • The early formation of structure; section 4 of Famaey & McGaugh.
  • Black holes are to inefficient at accreting; large black holes don’t have time to form in 13 billion years.
  • The ‘too big to fail problem’; M. Boylan-Kolchin, J.S. Bullock, M. Kaplinghat, “Too big to fail? The puzzling darkness of massive milky way subhaloes” Monthly Notices of the Royal Astronomical Society 415, p. L40, 2011.

From: Astronomy & Geophysics 51, Issue 5, pp. 5.14–5.16, October 2010:

  • The inclusion of a cosmological constant means that the ratio of the vacuum energy density to the radiation energy density after inflation is 1 part in 10100, a fine-tuning coincidence which leads to appeals to the anthropic principle for an explanation.
  • Even if fine-tuning arguments are regarded as unsatisfactory, the problem is that inflation was set up to get rid of fine-tuning in terms of the “flatness” problem and so the introduction of more fine-tuning with the cosmological constant appears circular.
  • Astrophysically, any cold-dark-matter (CDM) model in the first instance predicts a featureless mass function for galaxies, whereas the galaxy luminosity function shows a sharp “knee”.
  • CDM models predict that large structures should form last and therefore should be young whereas, observationally, the largest galaxies and clusters appear old.
  • To fix the above two problems, large amounts of feedback are invoked which results in more energy being used to prevent stars forming early than in forming them under gravity at later times.
  • Milky Way Galaxy not well understood; The Fermi Galactic Center GeV Excess arxiv:1704.03910, and the Fermi Bubbles arXiv:1802.03890.

From: M. López-Corredoira, “Non-standard models and the sociology of cosmology,”

  • Much higher abundance of very massive galaxies at high redshift than expected.
  • Wrong predictions at galactic scales (no cusped halos, excessive angular momentum, insufficient number of satellites, etc.). The cuspy core problem; W.J.G. de Blok, “The core-cusp problem” Advances in Astronomy 2010, p. 789293, 2010.
  • Dark energy in excess of theoretical models by a factor 10120.
  • Problems in understanding inflation.

© Louis Marmet 2020–2023