When we look at the dark sky on a night without clouds and far from city lights, we see plenty of stars like the Sun. Knowing that a substantial fraction of these stars host a rocky planet like the Earth at roughly the same separation implies that the Earth-Sun habitat is not unique and that suggests there might be exo-spectators like us who see our Sun from a distance. Our siblings in the extended family of intelligent beings hosted by the Milky-Way galaxy might look and think differently from us. Although we had never met them, we have kinship with them based on our shared cosmic experience.
Our nurturing cosmic neighborhood should not be taken for granted. In fact, most of the volume of the present-day Universe and half of its matter reside in nearly empty space.
The Universe started as a zero-sum game. Throughout most of its first 10 billion years, matter dominated the cosmic mass budget, and the matter density was just at the borderline of balancing cosmic expansion against gravity pulling matter together. This implies that small enhancements in the matter density triggered collapse and small deficiencies triggered enhanced dilution. The initial density perturbations left over from the Big Bang had equal probability of being positive and negative relative to the mean matter density. As a result, each point in space had a “toss of a coin” 50% likelihood of ending-up in a collapsed object or in a void. We ended up in a region which had a slightly higher density than average, which collapsed to make our Milky-Way galaxy, inside of which gas fragmented to make stars like the Sun. This should come as no surprise because our body’s density is 29 orders of magnitude larger than the mean density of the Universe, so it naturally assembled in an over-dense region.
We regularly focus our attention on stars and galaxies which illuminate the darkness of space. The matter that condensed to make these objects started in the intergalactic medium where its average density translates to one hydrogen atom per cubic meter, with a mass density that is 27 orders of magnitude lower than air. The material that made galaxies was drawn from voids which became emptier over time. We observe these voids in the distribution of galaxies on large scales and they stretch out to scales of tens of millions of light years, some with a present-day density below a tenth of cosmic mean value.
Initially, half of the volume of the Universe occupied under-dense regions that by now grew in size to become the voids we observe in the present-day Universe. The likelihood of having cosmic observers like us in voids is small, since they contain a small fraction of the Sun-Earth analogs in the Universe. For observers that were lucky enough to be born in a void, life must be boring. There are no nearby intergalactic hubs to meet aliens because matter is rarefied. Being born in a void can trigger melancholy. There is no escape from this sense of loneliness. Given the dominance of dark energy, voids grow larger rapidly as the cosmic expansion accelerates. They collide, fill up most of the cosmic volume and will become even emptier in the future.
Voids are dominated by the repulsive gravity of dark energy. As a result, the cosmic expansion of voids can be used to constrain dark energy. Since voids have less matter, the walls of their expanding bubbles tend to collide and coalesce in making even bigger voids over time.
The nature of dark energy and dark matter are unknown. The latest empirical constraints on cosmology from a combination of data from the DESI survey of the cosmic web of galaxies, the cosmic microwave background and supernova distances, was interpreted by the DESI team as implying an evolving dark energy with an equation of state that violated, about 10 billion years ago, the null energy condition of Einstein’s gravity. The null energy condition asserts that energy flowing slower than light must always be positive. Its violation, often labeled as a phantom behavior, creates instabilities and other problems in most extensions of the standard model of physics. In a new paper with my brilliant colleague, Xingang Chen, we explained the latest cosmological data by assuming an evolving dark matter instead of an evolving dark energy. Our theoretical model fits the data if a small component of dark matter has an oscillating equation of state within the physically acceptable range, without violating the null energy condition. From a fundamental physics perspective, this interpretation is more appealing than an evolving phantom dark energy. Additional data will test our hypothesis that dark matter is a mixed bag of different types of particles.
Galaxies represent islands of attraction in the vast expanse of cosmic space. Intergalactic travelers who are seeking company are likely to travel along the walls of the voids, where the most attractive tourist destinations are located. The galaxies on bubble walls are organized into filaments, which channel them towards clusters of galaxies. Altogether the initial 3D distribution of matter first collapses along one dimension into a 2D network of sheets that make bubble walls, which then drains the matter by collapsing along the second dimension into 1D filaments which eventually channel matter in the third dimension towards convergence points in the form of galaxy clusters.
Aside from hypothetical intergalactic tourists, weakly-interacting elementary particles also traverse the voids without inhibition. In particular, neutrinos free-stream in and out of voids owing to their small mass and weak interaction. A detailed study of the distribution of size and emptiness of voids can be used to constrain the sum of the masses of all neutrino species.
Voids behave like a portion of a Universe with a low matter density that will expand forever. Their evolution offers a glimpse at how boring our Universe would have been if its matter density was lower.
As with any other aspect of life, realizing environments with worse circumstances drives home the message of how grateful we should be for our cosmic fortunes. The Universe endowed us with the amazing opportunity to engage in social interactions with nearby aliens and we should take advantage of that while the Sun lasts. Even if there is “nothing new under the Sun” as argued in Ecclesiastes 1:9, there must be something new under the hundred billion other Suns within the Milky-Way galaxy.
We cannot let serendipity dictate our state of mind that we are alone, but instead seek intentionally other residents in our cosmic neighborhood. The fact that the first two interstellar objects, IM1 and `Oumuamua, appear anomalous is an encouraging sign that should make us curious about who else might be out there.
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.
