How Big is the Universe?



For now, it’s wondrous to know we live in a universe that’s at least 150 billion trillion miles across, and it may be much bigger than that. Since the Big Bang, the expansion of the universe has slowed and then sped up. In one research it was found that galaxies migrated apart slowly during the first half of cosmic history, and then a mysterious force — dark energy — accelerated the expansion.

Two great debates have taken center stage in the search to answer this age-old question. In April 1920, Harlow Shapley and Heber Curtis argued over the scale of the universe in the great auditorium of the Smithsonian institution’s Natural history museum in Washington. 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” — that the so-called spiral nebulae were distant systems of stars outside our milky way.

Shapley argued that spiral nebulae were merely gas clouds in the Milky Way. He further placed the Sun much of the way out near the edge of the galaxy — the entire universe, in his view — 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 in the galaxy, whereas Shapley was wrong about the smaller universe and right about the Sun’s location in it.

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. 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 constant (about 80 km/sec/mpc), suggesting a young age and correspondingly 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 set some limits on what must be true, based on the observations they have collected over the past century.

Using the most powerful telescopes now online, astronomers see galaxies 10 or 12 billion light-years away. (A light-year equals about 6 trillion miles or 10 trillion kilometers.) So the “horizon” of visibility is some 24 billion light-years in diameter. But that’s from our viewpoint. What about the horizons as seen from distant galaxies? It’s possible the universe is much larger than the portions we can see. This will be the case in the likely event that the inflation hypothesis, put forth by MIT’s Alan Guth, proves correct. This idea suggests the extremely young universe experienced a moment of hyper growth so severe it ballooned from the size of a subatomic particle to a softball’s size in an instant. 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 happened 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.

Below is a detailed diagrammatic analysis created by Robin Richards. It gives us quite an idea about the size of the universe.

Size of the Universe Analysis

Although the debate still continues whether the universe is dying or it still continues to expand.


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