[Paper Review] Low primordial information content in the Milky Way with warm dark matter

Mark C. Neyrinck|arXiv (Cornell University)|Aug 29, 2014

Statistical Mechanics and Entropy1 citations

TL;DR

This paper investigates the primordial information content of the patch that formed the Milky Way under warm dark matter (WDM) scenarios, finding that with a 1 keV WDM particle, the initial conditions would contain only ~1 billion independent 'pixels'—far fewer than the Milky Way's stars. This low information content challenges the idea that high galactic structure can emerge from smooth initial conditions, suggesting WDM with such low mass is disfavored under strong assumptions about primordial determinism and negligible non-primordial information sources.

ABSTRACT

We speculatively examine some issues related to the information content of primordial patches that collapse to form galaxies like the Milky Way. If the dark matter is warm, or if some other process dramatically suppressed small-scale density fluctuations, then the patch that formed the Milky Way would have had low primordial information content. Depending on assump-tions about the accuracy with which the initial conditions are specified, the patch would have contained only several billion independent information-carrying ‘pixels ’ if the warm-dark-matter (WDM) particle had a mass of 1 keV. This number of ‘pixels ’ is much less than even the number of stars in the Milky Way. Like other recent observational tests, this would provide an argument disfavoring such a low mass, under two strong assumptions: (1) a high degree of structure in the Milky Way cannot arise from very smooth initial conditions, and (2) non-primordial information/randomness sources are negligible. An example of a non-primordial information source is a central black hole with an accretion disk and jets, which in princi-ple could broadcast small-scale quantum fluctuations throughout the galaxy. This brings up a question, and even, in principle, a test (if the dark matter is warm) about the scale at which structure in the Galaxy is entirely deterministic from the initial conditions. Key words: large-scale structure of Universe – cosmology: theory

Motivation & Objective

• To assess the primordial information content of the initial patch that collapsed to form the Milky Way under warm dark matter (WDM) assumptions.
• To evaluate whether the observed high level of structure in the Milky Way can be explained by initial conditions with low information content.
• To test the viability of low-mass WDM (e.g., 1 keV) by comparing predicted initial information content with the complexity of the Milky Way.
• To investigate the role of non-primordial information sources—such as central black holes with accretion disks—in potentially masking or supplementing primordial structure.
• To explore whether structure in the Galaxy is fully deterministic from initial conditions under WDM scenarios, particularly at small scales.

Proposed method

• Modeling the initial density fluctuation patch that formed the Milky Way under the assumption of warm dark matter with a 1 keV particle mass.
• Estimating the number of independent information-carrying 'pixels' in the primordial patch by considering the suppression of small-scale density fluctuations due to free-streaming in WDM.
• Applying a theoretical framework that treats the initial conditions as a finite information source, with information content quantified in terms of degrees of freedom or independent modes.
• Comparing the estimated number of primordial 'pixels' to the number of stars in the Milky Way (~10^11) to assess whether such a low information content can account for observed galactic structure.
• Assessing the impact of non-primordial information sources—such as active galactic nuclei with jets and accretion disks—on the overall information content of the galaxy.
• Evaluating the determinism of galactic structure by asking whether all structure can be traced back to initial conditions under WDM, given the low initial information content.

Experimental results

Research questions

• RQ1 How many independent information-carrying 'pixels' would the primordial patch forming the Milky Way contain if the dark matter is warm with a 1 keV mass?
• RQ2 Can the observed level of structure in the Milky Way arise from such a low-information initial state under the assumption of primordial determinism?
• RQ3 To what extent do non-primordial sources—like central black holes with accretion disks—contribute to the galaxy’s information content and potentially undermine primordial determinism?
• RQ4 At what scale does the structure of the Milky Way become inconsistent with being fully determined by initial conditions under a low-mass WDM scenario?
• RQ5 Is there a testable distinction between primordial and non-primordial origins of galactic structure in the context of warm dark matter?

Key findings

• With a 1 keV warm dark matter particle, the primordial patch that formed the Milky Way would contain only approximately 1 billion independent information-carrying 'pixels'.
• This number of primordial 'pixels' is significantly smaller than the estimated number of stars in the Milky Way (~10^11), suggesting insufficient initial information to account for observed structure.
• The low information content challenges the viability of low-mass WDM (1 keV) if one assumes that high galactic structure cannot arise from very smooth initial conditions.
• The result is conditional on two strong assumptions: (1) that high structure cannot emerge from smooth initial conditions, and (2) that non-primordial information sources are negligible.
• Non-primordial sources—such as central black holes with accretion disks and jets—could in principle broadcast small-scale quantum fluctuations throughout the galaxy, potentially introducing structure independent of initial conditions.
• The study suggests a potential testable distinction between primordial and non-primordial origins of structure, particularly in the context of warm dark matter, by probing the scale at which structure becomes non-deterministic from initial conditions.