If we chose to shovel matter along a straight line through the observable Universe, what is the average mass of matter per unit area that our shovel will collect? To answer this question, I did a simple calculation before my morning jog at sunrise today.
The answer depends on how far the shovel goes. Projecting all the matter out to the farthest galaxy, MoM-z14, discovered last month by the Webb telescope at a cosmic redshift 14.44 or equivalently 280 million years after the Big Bang, the answer is about 0.5 grams per square centimeter, of order the mass per unit area of a thumb. This establishes the cosmic rule-of-thumb: the observable universe yields on average as much mass per unit area as a thumb.
This mass budget includes mostly dark matter whose nature is unknown. Ordinary matter accounts for only 16% of the total budget or 0.08 grams per square centimeter out to MoM-z14, of order the surface mass density of a fingernail.
Today’s cosmos is very rarefied. The average mass of matter per unit cosmic volume is only 2.8×10^{-30} grams per cubic centimeter, about 30 orders of magnitude lower than the mass density of familiar solids. The density is so low that even when integrating it over cosmic distances of billions of light years– the resulting surface mass density is minuscule.
Although matter is distributed nearly uniformly when averaged on large scales within the observable cosmic volume, much of it is clumped into objects of small spatial scales, like galaxies, stars and planets. Interestingly, the average mass per unit area through the core the Milky-Way galaxy is also of order a gram per square centimeter, with the distinction that the Galactic core is dominated by gas and stars rather than dark matter. The core size is of order ten thousand light years, a million times smaller than the scale of the cosmic horizon, but the core mass per unit volume is a million times larger than the cosmic average value, making the Galactic and cosmic surface densities similar.
This result is important for gravitational lensing of light by galactic cores, which depends on their projected mass per unit area. Placing the core of a galaxy like the Milky-Way halfway to the cosmic horizon, results in multiple images of a farther source of light, like a quasar. Multiple images emerge when the galaxy’s surface density exceeds a critical value for gravitational lensing. The critical surface density over cosmological distances happens to be about a gram per square centimeter, the same value that characterizes the cosmic rule-of-thumb.
Another interesting coincidence is that the primordial cosmic gas of free electrons and protons becomes opaque to light above a threshold surface density of 2.5 gram per square centimeters, about an order of magnitude above the cosmic rule-of-thumb value. This threshold surface density of ordinary matter is dictated by the ratio between the proton mass and the Thomson cross-section for scattering light by free electrons. This coincidence makes the gaseous cores of some galaxies opaque.
As explained in the textbook “The First Galaxies in the Universe”, that I wrote with my former student Steve Furlanetto, cosmic hydrogen was broken (re-ionized) by starlight from early galaxies at a redshift of about 6, where the cosmic surface density was well below 2.5 grams per square centimeters. Since ordinary matter makes only 16% of the mass budget of cosmic matter, the Universe remained mostly transparent all the way to the era of the early galaxies, allowing the Webb telescope to detect MoM-z14. The small electron opacity after reionization was measured to be about 6% by the Planck satellite, based on the related smoothing of the brightness fluctuations of the cosmic microwave background on scales smaller than the horizon at reionization. This relic microwave background radiation emanated 400,000 years after the Big Bang. Prior to that time, the primordial soup of free protons and electrons was sufficiently hot and dense so as to be opaque to primordial radiation. As a result, our telescopes cannot observe through the dense fog of electrons at earlier cosmic times.
Even though the average cosmic surface density is as low as that of a thumb, denser entities might be of greater interest to us. As I was having my early morning musings about cosmic surface densities, a reporter from the daily newspaper Večernji list in Croatia asked me to comment on the most exciting high-density entities that we might find: “What would happen if evidence of extraterrestrial life appeared on Earth? Or in space? How would world leaders react and what would it mean for world religions?” My response was as follows:
“The discovery of extraterrestrial life would have dramatic implications for humanity. The impact will depend on which form of life is found. If it involves microbes on the surface of Mars or in the clouds of Venus or under the surface ice on Europa or Enceladus or in the methane & ethane oceans of Titan, it will be of great scientific value. The most exciting follow-up question would be whether that form of life is made of the same genetic building blocks of life on Earth, which started from a single Last Universal Common Ancestor (LUCA), about 4.2 billion years ago.
But if the finding involves intelligent life beyond Earth, the impact would be much more dramatic, because it would imply that we might not be the smartest kid on the cosmic block. We might be able to learn from alien intelligence more than we can learn from our future artificial intelligence systems. The discovery will change our perspective on our place in the Universe and inspire space exploration. The discovery of sophisticated technological devices near Earth would mean that our governments may not be able to protect us from an extraterrestrial threat. This will restructure world governance and revise our state of mind, as we might realize that we are not at the top of the cosmic food chain.
Religious leaders will need to admit that God may not be parenting `a single child’ and that our civilization has siblings.”
After sending out my reply message, I had one remaining thought. As we all know from experience in blind dates, nearby objects with a large-surface density could be far more interesting than the rest of the Universe.
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.
