The Universe is Too Big for Us to Ignore It

Avi Loeb
5 min readSep 12, 2022
Theoretical illustration of the three-dimensional 21-centimeter mapping of cosmic hydrogen (Credit: J. Pritchard and A. Loeb 2011).

We live life with our attention mostly focused on the two-dimensional surface of Earth. On rare occasions space items creep into the news in the context of communication satellites, spaceflight missions to the Moon or other Solar system destinations, or new astronomical observations of more distant sources.

It would only be fair for our attention span to reflect the actual abundance of cosmic events, in which case we should focus on the Universe at large.

The limitation of human knowledge was acknowledged by Hamlet in William Shakespeare’s play:

“There are more things in heaven and Earth, Horatio

Than are dreamt of in your philosophy.”

How should we weigh our attention? As the Universe expands, matter gets diluted. But to get a fair assessment of the cosmic volume which deserves our attention, we can take out the expansion of the Universe and consider the so-called “comoving volume” inside of which the amount of matter remains fixed. This definition applies to a circle drawn on an expanding balloon, of which the area increases as the balloon inflates but the amount of rubber interior to the circle remains fixed.

Interestingly, three quarters of the comoving volume of the Universe resides at redshifts above 10 when the Universe was less than half a billion years old. This cosmic era is called the dark ages, followed by cosmic dawn when the first galaxies formed. The deep image of the Webb telescope, celebrated by President Biden and vice-President Harris at the White House on July 11, 2022, shows galaxies from that epoch.

If we wish to learn about the initial conditions of the early Universe, we better map most of this volume. In the local Universe, galaxies are used as tracers of the matter distribution on large scales. But mapping the matter distribution on large scales with early galaxies is not practical. There are two reasons for that. First, galaxies were rare early on. Second, they appear very faint to our telescopes because the first dwarf galaxies had low masses and also because they are located at large distances dictated by their early formation times. A much better approach is to map the cosmic gas between the galaxies directly.

Fortunately, hydrogen atoms display a radio transition at a wavelength of 21-centimeter associated with a spin flip of the electron relative to the proton at the ground state. In 2004, I wrote a paper with Matias Zaldarriaga showing that most of the information about the initial conditions of the Universe can be retrieved from three-dimensional 21-cm mapping of primordial hydrogen at early cosmic times. The observed wavelength of the radiation is stretched by the cosmic expansion and flags the third dimension of cosmic time in addition to the two-dimension of the sky coordinates. The primordial radiation of the cosmic microwave background maps the distribution of matter on a two-dimensional “last scattering surface” of the photosphere that emitted it at redshift 1,100, merely 400,000 years after the Big Bang. There is much more information in the three-dimensional volume available for us to map since that time when the Universe was transparent. In a paper from 2012, I showed that the optimal cosmic epoch for 21-cm mapping of the cosmic initial distribution of matter is around a redshift of 10, half a billion years later.

Most people care more about life than about dark matter. In that case, we should weigh our attention based on stars because their light and heavy element production enabled life-as-we-know-it. This implies that we should focus on the last 10 billion years, since the observed star-formation history of the Universe implies that most stars formed at redshifts below 2.

Of course, there is a limit to how much we can learn since we can only look out to the distance that light travelled since the Big Bang. From the uniformity of matter inside our horizon, we can conclude that there is no big change in the distribution of matter on scales that exceed our cosmic horizon by approximately a factor of 4,000. For now, we only surveyed a tiny fraction of the available archaeological dig of the cosmos.

It is remarkable that the biblical story of genesis has a beginning in time similar to the Big Bang model, and the verse “Let there be light” can be broadly interpreted in the context of the microwave background or the emergence of light from the first stars. I wrote two textbooks on the first stars and galaxies (here and here), but I would be particularly pleased to see surprising data from the Webb telescopes that would revise some of the expectations reviewed in my books. This experience will follow tradition, as the details of the cosmological sequence of events observed through our telescopes deviate from traditional stories.

Scientific knowledge is an island in an ocean of ignorance. Expanding the landmass of this island is work in progress. At times we try to guess what lies under the ocean but as the water recedes and we expand our knowledge, we often find that we were wrong. We are all students of nature and we must seek to learn from it by humbly collecting evidence on where we actually live instead of building fictitious castles of virtual realities that flatter our ego.


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 chairs the advisory board for the Breakthrough Starshot project, and 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.



Avi Loeb

Avi Loeb is the Baird Professor of Science and Institute director at Harvard University and the bestselling author of “Extraterrestrial” and "Interstellar".