So one of our readers asked us this question the other day. Is the expansion of the universe accelerating?

Short answer: Yes.

Edwin Hubble

Long answer: Edwin Hubble (the namesake of the Hubble Space Telescope) observed that distant galaxies were moving away from us. More importantly, he noticed that the speed of their recession increased linearly with distance. This rule that “Twice as far means twice as fast” is Hubble’s law.

Distant Galaxies

Hubble’s original observations were very rough; he concluded galaxies were moving away at 500(km/s)/Mpc (we now know this number is closer to 70 (km/s)/Mpc). What this means is that for every megaparsec (about 3 million light years) of space between us and a distant galaxy another 70 kilometers of space get ‘stretched into existence’ between us every second. Hubble’s law is a very good law for describing the motion of galaxies that are over 100 million light years away, and up to a few billion light years away.

To study the acceleration of the expansion, we have to look at how the expansion changes in time, and to do that, we have to look farther away. The effect of the acceleration is tiny, and can really only be observed when looking at literally the other side of the universe.

Distant supernova

In the 1990s some scientists observed very very distant supernova in the universe. These were a specific type of supernova that have a uniform brightness, which allowed them to find the distance to the supernova based on their apparent brightness. When they observed the supernova’s redshift (which tells us their recession velocity) and brightness (which tells us their distance), they found that the supernova were moving slower than we would expect based on their distance.. This tells us that the universe wasn’t expanding as quickly in the past as it is now, hence it is accelerating.

These scientists won the Nobel prize in 2011.


Just a side note. Don’t think of distant galaxies as moving through space, because they aren’t. Think instead of changes in geometry over time, because that’s what’s happening. When we talk about accelerated expansion, we’re talking about the way the rate of change in geometry changes with time.

The essence of it is that the distances between fixed points in the universe are increasing over time. Take any two points, measure the distance between them, then wait a reasonable amount of time — say a dozen billion years. Measure the distance again and you’ll find that the distance has increased. The two points are not moving. But the distance between them is not fixed.

So when viewed from a single point at a single instant, it appears that objects sitting out in space at those fixed points are receding from us, and that their speed of recession is proportional to how far away they are. But we know that isn’t the case. It’s just an optical illusion.

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Last Update: April 25, 2016

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