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James S. Bullock, Michael Boylan-Kolchin | Annual Review of Astronomy and Astrophysics | (2017)
Primary Link DOI Openalex Full text (OA)

Key Takeaways

Sample Definition And Size

This is a narrative review article, not an empirical study; it does not involve a specific sample size or number of subjects. Instead, it reviews theoretical and observational challenges to the ΛCDM paradigm on small scales, drawing on existing literature and simulations. No meta-analysis of a defined number of papers is conducted.

Study Type

Review article (Annual Review of Astronomy and Astrophysics, 2017), summarizing theoretical and observational challenges to ΛCDM on small scales.

Conflicts Of Interest

The authors declare they are not aware of any affiliations, memberships, funding, or financial holdings that might be perceived as affecting the objectivity of this review. ([ned.ipac.caltech.edu](https://ned.ipac.caltech.edu/level5/Sept18/Bullock/Bullock5.html?utm_source=openai))

Results Summary

The review identifies three main small-scale challenges to ΛCDM: (1) the cusp/core problem—observed dark matter–dominated galaxy cores are less dense and less cuspy than predicted; (2) the missing satellites problem—the observed number of dwarf galaxies in the Local Group (~50 known satellites) is far below the thousands of predicted low-mass dark matter subhalos; (3) the too-big-to-fail problem—simulations predict massive subhalos that should host visible galaxies, yet such dense satellites are not observed. Additional anomalies include planar and orbital configurations of Local Group satellites and tight baryonic/dark matter scaling relations. The review emphasizes that baryonic physics or new dark matter physics may resolve these issues, and highlights future observational programs (e.g., LSST era surveys) to detect faint dwarf galaxies and characterize dark subhalos as critical tests of ΛCDM. ([annualreviews.org](https://www.annualreviews.org/eprint/GVBsUb4zsRIRDHhaz6Gz/full/10.1146/annurev-astro-091916-055313?utm_source=openai))

Abstract

The dark energy plus cold dark matter (ΛCDM) cosmological model has been a demonstrably successful framework for predicting and explaining the large-scale structure of the Universe and its evolution with time. Yet on length scales smaller than ∼1 Mpc and mass scales smaller than ∼10 11 M ⊙ , the theory faces a number of challenges. For example, the observed cores of many dark matter–dominated galaxies are both less dense and less cuspy than naïvely predicted in ΛCDM. The number of small galaxies and dwarf satellites in the Local Group is also far below the predicted count of low-mass dark matter halos and subhalos within similar volumes. These issues underlie the most well-documented problems with ΛCDM: cusp/core, missing satellites, and too-big-to-fail. The key question is whether a better understanding of baryon physics, dark matter physics, or both is required to meet these challenges. Other anomalies, including the observed planar and orbital configurations of Local Group satellites and the tight baryonic/dark matter scaling relations obeyed by the galaxy population, have been less thoroughly explored in the context of ΛCDM theory. Future surveys to discover faint, distant dwarf galaxies and to precisely measure their masses and density structure hold promising avenues for testing possible solutions to the small-scale challenges going forward. Observational programs to constrain or discover and characterize the number of truly dark low-mass halos are among the most important, and achievable, goals in this field over the next decade. These efforts will either further verify the ΛCDM paradigm or demand a substantial revision in our understanding of the nature of dark matter.

Referenced In

StarTalk

Season 16 Episode 76 – A Short History of Expanding Universe Models

Hey StarTalkers! Following up on the previous post, this post delves a little deeper into the historical impact of Professor Adam Riess’s research and the discussion in Season 16, Episode 76.

Discovering Invisible Forces in Our Universe, with Adam Riess

Professor Riess’s work was the final nail in the coffin of many older cosmological models, proving that dark energy exists and issuing in a new era of cosmology. The ΛCDM model became the dominant model of cosmology and – despite some issues – it remains that way today.

Einstein’s Static Universe

Einstein basically kick-started modern theoretical cosmology in his 1917 paper based on his theory of general relativity. He used the “cosmological constant” to make the universe static, balancing out the effect of gravity.

This had a lot of problems, though. Just after publishing, Einstein’s friend Willem de Sitter pointed out a major issue. Even in a completely empty universe, it predicted that a “test particle” would move. He asked, rhetorically, “has this inertia?”

The “Steady State” Universe

In 1948, Hermann Bondi, Thomas Gold and Fred Hoyle invented what’s called “steady state cosmology.” In their model, the universe expands, but it rests on the assumption of the “perfect cosmological principle.” This means that the universe has to look the same on large scales at all times.

With an expanding universe, the only way this is possible is if there is a constant influx of new matter. And that’s what they did – they proposed a “creation field” which churned out new matter as needed.

Notably, they included the cosmological constant as a fundamental constant of nature.

The Expanding Universe

Models of the universe involving expansion first got going with Alexander Friedman in 1922, and Georges Lemaître later discovered the same solutions to Einstein’s field equations. In 1928, Lemaître and Howard Robertson made an initial estimate for the speed of expansion.

Just a year later, Edwin Hubble would publish his famous equation linking the observed redshift of cosmic objects to their distance from the observer.

This made it clear that the universe really was expanding, and attention shifted accordingly. Even Einstein got involved again in 1932, proposing the “Einstein-de Sitter” model, which removed the cosmological constant, incorporated expansion and arguably even predicted dark matter.

But Adam Riess’s 1998 discovery showed that these early pioneers didn’t go far enough. Not only is the universe expanding, the expansion is accelerating.

ΛCDM to the Rescue?

After this long, winding journey, cosmologists have settled on the ΛCDM model of the universe. This includes the cosmological constant – Λ, lambda, to account for Riess’s result – and also “cold dark matter.” This has been very successful, overall, but issues like “Hubble tension” persist.

But one thing still seems clear: the expansion of the universe – dark energy – is real.  

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