How Did Scientists Determine The Size Of The Universe?

Based on various measurement tools at different ranges of distance (trigonometry, parallax, standard candles, supernovae brightness, galactic red shift and the cosmic microwave background), it is possible to create a cosmic distance ladder and accurately determine not only the distance of far-flung galaxies, but also the current size of the universe.

When some people stare up into the night sky, it can be an incredibly peaceful experience – one that inspires a sense of clarity (or insignificance). However, for those endlessly curious people on this planet, looking out into the stars seems to bring back an endless stream of questions and mysteries. What happens on the other side of an event horizon? What is dark matter and what the heck does it do? How big is the universe?

That final question (at least out of those three) is something that we can answer, although the explanation isn’t the easiest thing in the world to understand. Without getting into too many tangential or peripheral topics in astrophysics, let’s try, as simply as possible, to answer that question: how do scientists know how big the universe is?

Short Answer: Based on various measurement tools at different ranges of distance (trigonometry, parallax, standard candles, supernovae brightness, galactic red shift and the cosmic microwave background), it is possible to create a cosmic distance ladder and accurately determine not only the distance of far-flung galaxies, but also the current size of the universe.

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How Are Distances In Space Measured?

Before we can get into the most massive distances in the universe – billions of light-years – it is important to start at the bottom of the cosmic distance ladder.

Up Close And Personal

When we are attempting to measure things that are relatively close, such as things within our solar system and even in our small “neighborhood” of the Milky Way (within 100 light-years), it is possible to use basic trigonometry, just like you learned (and promptly forgot) in high school math class. Basically, if you measure a star’s location at one point in the sky during the year, and then measure its position six months later, you will have a relative position of the nearby object in relation to the far more distant stars in the sky.

If you know the size of the Earth’s orbit, then based on the angle of light in those two measurements, it will be possible to calculate the distance (Thanks, Pythagorean Theorem!). However, the further away a star is, the less of a shift it will have, making the measurement of this angle impossible, which means the next rung in the ladder is required…

Standard Candles

Once you get beyond the range of trigonometry as a useful tool, astronomers rely on stars called Cepheids, which are quite common and very bright. The first Cepheid variables (Eta Aquilae and Delta Cephei, the prototype of the class) were spotted in 1784 by the English amateurs Edward Pigott and John Goodricke. These particular stars have the tendency to pulsate, growing dimmer and brighter in a regular pattern. Interestingly enough, the longer a Cepheid takes to pulse, the brighter the star actually is, while the shorter the pulsation period, the dimmer the Cepheid is.

This neat relationship between period and brightness, called the period-luminosity relation, was nailed down by Henrietta Swan Leavitt at the Harvard College Observatory in 1912, after she patiently catalogued 25 Cepheids in the Small Magellanic Cloud. Her work is what turned Cepheids from astronomical curiosities into yardsticks. By measuring nearby Cepheids with the parallax method (explained above), and then comparing the length of their pulsation period to those Cepheids that are further away, the true brightness of those stars can be determined, and thus their distance can be calculated. Modern Cepheid measurements with the Hubble Space Telescope and James Webb Space Telescope now reach galaxies as far as 40-50 megaparsecs (~130-160 million light-years), forming a critical rung of the cosmic distance ladder.

Supernovae

Even 160 million light-years is only a tiny slice of the cosmos, which is hundreds of times bigger, so yet another rung on the ladder is required. Supernovae come in extremely handy here, primarily those associated with binary star systems containing a white dwarf. In the simplest "single-degenerate" picture, one star in the pair has already died and become a white dwarf, which then begins to feed off its companion, growing closer and closer to about 1.4 times the mass of our Sun (the Chandrasekhar limit). Astronomers now think many Type Ia supernovae also come from "double-degenerate" pairs, where two white dwarfs merge, so the binary is essentially destroyed rather than half-surviving, but the trigger mass is similar.

Either way, the end is the same: a massive thermonuclear explosion, so bright it can be seen across half the observable universe, briefly outshining entire galaxies. These are known as Type Ia supernovae. Because the trigger mass is roughly fixed, the explosion's intrinsic brightness is roughly fixed too, so the dimmer it appears from Earth, the further away it must be. That lets astronomers calculate distances to far-flung galaxies.

Redshift

At even greater distances – into the tens of billions of light-years – something called the Hubble Constant comes into play. Named after Edwin Hubble, this is the unit of measurement for the expansion of the universe. Now, this is where this entire conversation gets a bit more confusing. This might be difficult to comprehend, but not only is the universe expanding in all directions at the same time, at an ever-increasing rate, but the space between different objects in the universe is also expanding. This inter-object expansion (and acceleration) is related to dark energy, which goes beyond the scope of this article, but suffice to say, everything is moving away from everything else, and it is moving faster and faster all the time.

The exact value of the expansion rate is actually one of the hottest open problems in cosmology right now. The "local" distance ladder (Cepheids plus Type Ia supernovae, in the SH0ES program) currently gives a value of about 73 kilometers per second per megaparsec (3.26 million light-years), while the value inferred from the cosmic microwave background by ESA's Planck mission is about 67.4 km/s/Mpc, and recent DESI baryon-acoustic-oscillation results land near 68.5. The disagreement, known as the "Hubble tension," is over 5 sigma and nobody is quite sure why. Either way, the further away you look, the faster the galaxies are moving away from us; the redshift of a galaxy's light tells you how fast it's receding.

The first redshift measurements of "spiral nebulae" (which turned out to be galaxies) were actually made by Vesto Slipher at the Lowell Observatory starting in 1912. Hubble's contribution was to combine those redshifts with Cepheid distances and, in 1929, show that redshift scales with distance, the now-famous Hubble (or Hubble-Lemaitre) law. The principle is straightforward: when light comes from an object moving away from you, its wavelengths get stretched and the spectral lines slide toward the red end of the spectrum, while light from an object moving toward you gets compressed into a "blueshift." For the vast majority of galaxies, the light is shifted red, meaning everything is rushing away. By plotting redshifts against known distances, astronomers can extend the cosmic distance ladder out to the largest scales we can see.

Cosmic Microwave Background

This is the final measurement tool, and basically functions as a baseline for distances within the universe. The cosmic microwave background (CMB) is the leftover radiation from the very earliest moments following the Big Bang (earliest is a relative term, and is roughly 380,000 years after the Big Bang, when the universe finally cooled enough for atoms to form and light to travel freely). This radiation is the oldest and furthest radiation ever detected, filling the "empty" space between stars. Combined with everything else, it puts the age of the universe at about 13.8 billion years (Planck's best value is 13.787 plus or minus 0.020 billion years).

That essentially means the furthest back in time we can see is about 13.8 billion years, because that's how long the light has had to travel to reach us. Naively, then, the "observable universe" has a look-back radius of 13.8 billion light-years and a look-back diameter of roughly 27.6 billion light-years.

However, the actual present-day "observable universe" is bigger than that. Think back to the Hubble Constant, and its measurement of the expansion of the universe. The CMB photons reaching our eyes right now have been traveling for 13.8 billion years, but the universe has been expanding the entire time, pushing the "edge" of what we could ever have signal from much further out. Plug in the Hubble Constant and you can calculate where the original source of those photons is now: roughly 46.5 billion light-years away. That makes the observable universe about 93 billion light-years across.

Clearly, calculating distances of this magnitude can bend the brain beyond the realm of human comprehension. Fortunately, with the cosmic distance ladder in place, and plenty of astronomers well versed in these types of measurements, we’re able to keep tabs on how the universe is moving – as well as our place within it!