How Astronomers Know the Universe Is Expanding

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Here’s another ‘Ask the Expert’ question from our readers:

“So the usual response to why the universe is definitely expanding is the relative acceleration of everything in our frame of reference. But presuming there is a central body of mass in the universe, wouldn’t the closer objects accelerate towards this more quickly, hence creating the same relative acceleration? What additional evidence invalidates this possibility?”

Let’s start with an explanation of how astronomers know that the universe is expanding. The concepts we need to look at are velocities – not accelerations – of distant galaxies.

How Hubble Discovered the Expanding Universe

Here, we reference information previously provided in another article about the expansion of the universe here on Decoded Science:

Beginning in the 1920s, Edwin Hubble used the 100 inch telescope on Mount Wilson to measure the distances to galaxies using Cepheid variable stars as distance indicators. Hubble also studied the spectra of galaxies and found that all but the nearest galaxies had a Doppler redshift in their spectra. Doppler redshifts tell us that an astronomical object is moving away from us. Larger redshifts indicate larger recessional velocities, or objects that are moving away faster. 

When Hubble graphed the recessional velocities of galaxies versus their distances, he found a linear relationship. The more distant galaxies appeared to be moving away from us more rapidly. Astronomers call this graph the Hubble plot or Hubble’s law.

Hubble then deduced the reason for this effect:  The universe is expanding. The slope of the Hubble plot, called the Hubble constant, tells astronomers how fast the universe is expanding and allows astronomers to calculate the age of the universe in the context of the big bang model. Astronomers also use Hubble’s law to calculate the distances to very distant galaxies from their measured redshifts.

(2017 Note: Hubble measured recessional velocities rather than accelerations when he discovered that the universe is expanding)

To visualize why Hubble’s law tells astronomers the universe is expanding, draw a bunch of dots on a balloon. Then watch the dots while blowing up the balloon. From the vantage point of any given dot all the other dots will appear to be moving away with the more distant dots moving away more rapidly. Graphing the apparent recessional velocities of the dots on the balloon versus their distances will give a graph very similar (on a much smaller scale) to Hubble’s plot.

Now that we’ve established some background on this topic, let’s get back to our reader’s specific question – but keep that balloon on hand!

Formal portrait photograph of Edwin Hubble in 1931 Image Credit: Johan Hagemeyer.

Does the Universe Have a Central Point or Mass?

Now, as you are blowing up the balloon, think about this: Is there a central point to the balloon on the surface of the balloon?

Note that measuring the recessional velocities as seen from any one of the dots produces the same result. There is no special central point on the surface of the balloon. The center of the balloon is actually inside the balloon, rather than on the two dimensional surface of the balloon.

Similarly there is no center to the three-dimensional universe. Therefore there is no central mass in the universe.

Both our solar system and our galaxy have central points. The central mass in the solar system is the Sun, and there is a central black hole in the Milky Way galaxy. There is, however, no corresponding central point or mass for the entire universe.

Are Distant Galaxies Accelerating?

Note that it is not the acceleration of distant galaxies that tells us that the universe is expanding. Rather it is the recessional velocity of these distant galaxies and the fact that more distant galaxies have greater recessional velocities.

Astronomers have not measured any accelerations (defined as any change in velocity) for distant galaxies. Such a measurement would require a prohibitively long time. There is, however, a famous discovery that might be confused with the acceleration of distant galaxies.

The slope of the Hubble plot tells astronomers the rate at which the universe is expanding. The steeper the slope, the faster the universe is expanding. This slope is approximately but not exactly constant over time. The 2011 Nobel Prize in Physics was awarded to Saul Perlmutter, Brian Schmidt, and Adam Riess for their discovery that the rate at which the universe is expanding is accelerating. The slope of the Hubble plot differs slightly for the most distant galaxies.

The rate at which the universe is expanding is accelerating, but individual galaxies are not accelerating. Astronomers do not know what causes this acceleration. They call it the ‘dark energy’ but do not know what the dark energy is.

Universe Expansion: Closing Notes

Astronomers know that the universe is expanding because Hubble observed that more distant galaxies have larger recessional velocities. The fact that the universe is expanding does not require that the universe has a center point or central mass. Astronomers have observed no changes in the recessional velocities of individual galaxies; hence, individual galaxies are not accelerating. The rate at which the entire universe is expanding is, however, accelerating.

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