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The lingering imprint of the first cosmic structures

February 01, 2023

The universe today is host to a vast network of galaxies and an even richer array of invisible dark matter structures. But this was not always the case. The universe was nearly uniform until a time of about 100 million years, when the first cosmic structures gravitationally condensed. These objects were made of dark matter alone and each may have weighed no more than the Earth. Most of these objects do not last long: they rapidly grow and cluster together to form the much larger systems that we know today. Despite this, scientists at MPA have discovered in high-resolution simulations that some unique features of the first structures survive this process. Their lingering imprint could manifest itself in astronomical observations, yielding clues to the identity of dark matter.

About 85% of the matter in the universe is invisible dark matter. While the microscopic nature of dark matter remains elusive, its gravitational influence is well understood. Dark matter drives the formation of all cosmic structure. The galaxies that we observe in the sky lie at the centers of much larger halos of dark matter. In addition, a vast multitude of smaller dark matter halos are believed to exist, too small to hold visible material.

The early universe looked very different. The distribution of matter was smooth and nearly uniform. Over time, gravity amplified the initial, minuscule variations in the density of the universe. Regions of excess density gradually pulled in surrounding material, becoming still denser. This process eventually culminated in gravitational collapse, which created the first dark matter halos (see movie).

Formation of a prompt cusp

The formation of a first-generation dark matter halo. At first, there is only a region of excess density, but its gravitational pull eventually causes it to collapse into a halo (at time t/tc=1). The figure on the right shows how the internal density of the system depends on the distance from its center. After collapse, a “prompt cusp” forms in which the density scales as the -1.5 power of the radius (see dashed line as reference).

Simulations of the gravitational collapse process show that it yields a remarkable feature. Plotting the density inside the resulting halo as a function of the distance from the center of the system, the researchers found a power-law dependence. This kind of mathematical relationship appears as a straight line in the logarithmic plot of density against radius, once the matter has condensed towards the center of the system. Moreover, it is evident from the movie that the density “cusp” forms promptly upon gravitational collapse and remains stable after its formation. Over longer simulated timescales, the researchers continued to find no evidence that these prompt cusps were significantly altered by growth of the dark matter halos around them, even as these halos grew a thousand-fold.

The number of halos today is expected to be astronomical. There may be thousands of them for every solar mass of dark matter, with almost all of them being far too small to hold any visible matter. Yet every halo formed from a first-generation object. If prompt cusps survive, every halo should have one at its center. Our own Milky Way galaxy and its dark matter halo may contain over a quadrillion smaller halos. This would mean that prompt cusps of dark matter outnumber stars in the galaxy by a factor of ten thousand!

This has significant implications in the search for signs of dark matter’s microscopic identity. Dark matter must have formed in the universe somehow, and one of the most widely studied ideas is that it could have been produced in particle-antiparticle pairs in the very hot, very early universe. If the dark matter was produced in this way, then dark matter particle-antiparticle pairs can annihilate today into detectable radiation. A variety of “indirect detection” experiments are searching for this annihilation radiation.

But dark matter annihilates more efficiently inside denser regions. Within prompt cusps, the density is extraordinarily high. MPA scientists estimate that the enormous abundance of these features could raise the predicted dark matter annihilation rate tenfold, a conclusion that significantly alters what our observational data tell us about the nature of dark matter.

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