From the July 2019 issue

How Big is the Universe?

Two great debates have taken center stage in the search to answer this age-old question.
By | Published: July 1, 2019 | Last updated on May 18, 2023
In 2014, when astronomers added ultraviolet data to the Hubble Ultra Deep Field, they were able to study some of the youngest, largest, and hottest stars located between 5 billion and 10 billion light-years from Earth, ultimately finding that young galaxies grow by forming small groups of very hot stars. In this deep portrait of the universe, we see roughly 13 billion years of cosmic history.
In April 1920, Harlow Shapley and Heber Curtis argued overthe scale of the universe in the great auditorium of the Smithsonian Institution’s Natural History Museum in Washington, D.C. 

In this discussion, which preceded Edwin Hubble’s discovery of the nature of galaxies by just a few years, Curtis argued that the cosmos consists of many separate “island universes,” claiming that the so-called spiral nebulae were distant systems of stars outside our Milky Way. Meanwhile, Shapley argued that spiral nebulae were merely gas clouds in the Milky Way. Shapley further placed the Sun toward the edge of our galaxy — which, in his view, was the entire universe — whereas Curtis believed the Sun to be near the galaxy’s center. Curtis was right about the large size of the universe but wrong about the Sun’s place within it. On the other hand, Shapley was wrong about the small size of the universe but right about the Sun’s location within it. 

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With the advent of many extragalactic distance measurements and two camps arguing for different results on the critical number called the Hubble constant — the expansion rate of the universe — astronomers staged a second great debate in 1996. The age and size of the universe are, of course, interrelated, and both depend critically on the Hubble constant.

Faint Mysteries
Strange, young star-forming galaxies (circled) in this Hubble Space Telescope image from 2009 are less than a billion years old
NASA/ESA/G. Illingworth and R. Bouwens (University of California, Santa Cruz)/The HUDF09 Team
In the same auditorium used by Shapley and Curtis, galaxy researchers Sidney van den Bergh and Gustav Tammann argued over the question. Van den Bergh offered evidence supporting a high value of the Hubble constant (about 80 kilometers per second per megaparsec), suggesting a young age and therefore small size of the universe. Tammann argued for a low value of the constant (about 55 km/sec/Mpc), which would indicate an older, larger universe. 

As was the case with Shapley and Curtis, the antagonists van den Bergh and Tammann each provided crisp, clear-cut arguments and data supporting his side, and neither succeeded in convincing astronomers from the other camp. As yet, astronomers are limited by both assumptions and a lack of adequate data to agree on the cosmic distance scale.

Despite this, astronomers can still set some limits on what must be true based on the observations they have collected and refined over the past century. Using today’s most powerful telescopes, astronomers see galaxies located over 13 billion light-years from Earth. (A light-year equals about 6 trillion miles, or 10 trillion kilometers.) Since they see these distant galaxies in all directions, the current “horizon” of visibility is at least 26 billion light-years in diameter.

Since the Big Bang, the expansion of the universe has slowed and then sped up. In this illustration, concentric red circles show that galaxies migrated apart slowly during the first half of cosmic history, and then a mysterious force — dark energy — accelerated the expansion.
NASA/Ann Feild (STScI)
But the universe is probably much larger than the portion we can see. This will be the case in the highly likely event that the inflation hypothesis, put forth in 1980 by MIT’s Alan Guth, proves correct. This idea suggests that the extremely young universe experienced a brief period of hypergrowth so severe that it ballooned from the size of a subatomic particle to the size of a softball almost instantly. If inflation occurred, then the universe is much larger than we might expect based on current observations. 

Here’s where it gets weird: If inflation happened, then it may have occurred in many places (perhaps an infinite number) beyond the visible horizon and the limits of the space-time continuum we are familiar with. If this is so, then other universes might exist beyond our ability to detect them. Science begs off this question, as by definition science is about creating and experimenting with testable ideas. For now, it’s wondrous enough to know we live in a universe that’s at least 550 billion trillion miles across, and it may be much bigger than that