Seventy percent of the cosmic energy budget is currently in the form of dark energy. The repulsive gravity induced by dark energy drives the accelerated expansion of the Universe. The nature of dark energy is unknown but its properties can be inferred through its impact on the history of cosmic expansion.
The latest information from the DESI collaboration was recently documented in a set of papers which provided a new analysis of dark energy based on maps of the spatial distribution of nearly 15 million galaxies and quasars. The DESI observations were combined with data from studies of the cosmic microwave background, supernovae, and weak gravitational lensing. The standard model of cosmology struggles to explain all the observations when taken together, but a model in which the dark energy density had an unusually exotic negative pressure when the Universe was a few times younger fits all current data well.
The implied negative pressure of dark energy in the early Universe exceeds in magnitude its energy density. Such an exotic substance violates the so-called “null energy condition,” which asserts that energy cannot be negative even if it flows at the speed of light. Although such a substance is not forbidden, it often leads to theoretical pathologies, such as instabilities stemming from it having no minimum energy state as well as having ghost states which are unphysical. Given these problems, one may wonder whether it is possible to assign the exotic nature implied by DESI from the dark energy sector to the dark matter sector. In the early Universe, dark matter was denser and constituted a larger fraction of the cosmic budget than it does today. Assigning a negative pressure to it could in principle account for the anomaly implied by the latest DESI analysis.
Whether a significant fraction of dark matter could have negative pressure is a question that I discussed over the past week with my brilliant colleague, Xingang Chen. This morning, before my jog at sunrise I realized that such a possibility is not allowed by what we know about the Universe.
As I found out, dark matter with a large negative pressure would be extremely unstable to gravitational fragmentation. It would have quickly produced collapsed objects on all scales out to the cosmic horizon.
It is easy to understand the origin of this fragmentation instability. A small enhancement in the density of exotic dark matter would have resulted in a reduction of its pressure to an even more negative value. This implies that its larger outside pressure would have pushed more exotic matter inwards and make the central pressure even more negative. This runaway process triggers an instability which is driven by the negative pressure.
In the standard cosmological model, the clumping of matter is, however, driven by gravity and suppressed by positive pressure — as described in detail in my textbook “The First Galaxies in the Universe”. The standard growth of collapsed objects out of initial density perturbations occurs on a timescale which is comparable to the age of the Universe.
However, for exotic dark matter with a large negative pressure, the fragmentation could be much faster. The corresponding growth is exponential in time and occurs over a timescale of order the light-crossing time of the length scale of the collapsing region. This timescale is shorter than the age of the Universe by the ratio between the scale of the object and the scale of the cosmic horizon.
As a result of this instability, exotic dark matter with a large negative pressure would have quickly collapsed into objects that could be as big as the cosmic horizon. This expectation is in conflict with the observed properties of the Universe.
The largest collapsed objects are observed to be on the scale of clusters of galaxies whose size is smaller than the cosmic horizon by a factor of ten thousand. If larger collapsed objects existed, they would have imprinted a large anisotropy in the brightness of the cosmic microwave background which is not detected in the real Universe.
Exotic dark matter with a large negative pressure is ruled out by the observed large-scale structure of the Universe, which features only small density fluctuations on scales much bigger than clusters of galaxies. The fluctuations are of order of one part in a hundred thousand on the scale of the cosmic horizon.
The fragmentation of exotic dark matter relies only on its large negative pressure and is not influenced by gravity from other constituents of cosmic matter which are slower to collapse.
In conclusion, the exotic properties inferred by DESI are more likely to apply to dark energy than dark matter. The true nature of dark energy is yet to be discovered.
ABOUT THE AUTHOR
Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. The paperback edition of his new book, titled “Interstellar”, was published in August 2024.
