How do we know that the universe is expanding?

Let’s start with a little experiment that will give us a picture of an “expanding universe”. This universe will be an inflatable balloon.

We mark with a pen any point on the surface and draw a small circle around it and mark two points on the circle. The balloon is gradually inflated.

Seen from any point on the surface, all other points retreat as if it were the center of enlargement.
Jacques Treiner, Provided by the author

As the circle grows, so does the distance to the center, as does the distance between the two points on the circle. This applies regardless of the chosen starting point. To get a picture of an expanding universe, it is sufficient to generalize the case of a surface to the case of a volume. Each point “sees” the other points move away from it, as if it were the center of enlargement.

Large-scale extension, but not necessarily local

Now we need to explain how scientists came to this conclusion regarding the observable universe, and not just an inflatable balloon.

For this we must observe the universe on a large scale. Neither the Moon nor the Sun moves away from the Earth, nor do other objects in the solar system. The stars in our galaxy, the Milky Way, are not moving away from us. And even the Andromeda Galaxy, which is more than two million light-years (AL) away, is not moving away from us. On the contrary, it is approaching us at a speed of 500 km per second.

Is the universe really expanding? Yes, but on scales of tens, hundreds of millions and billions of AL. On average, galaxies move away from each other, but that does not stop some from getting closer locally, and even colliding.

A collision of galaxies
Example of a collision of galaxies: The Muse galaxy, located 301 million AL from our galaxy.
William Ostling / NASA

We have known about the expansion of the universe since the 1920s, when astronomers (Americans, in this case) observed that distant celestial bodies were moving away from us and that their rate of removal was greater the further apart they were. To do this we had to be able to measure, for each object, its distance from us and its velocity.

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Speed ​​measurement

The turning point came when physicists analyzed light coming from stars, beginning with the Sun. Newton realized that white light consisted of a continuum of wavelengths, but that was not until the early 1900s.e century that Frauenhoffer, a German physicist, noticed the presence of dark lines in the solar spectrum.

Sunlight spectrum with dark areas
Dark lines on a continuous solar spectrum.
Jacques Treiner, Provided by the author

These “absent” wavelengths are due to their absorption of elements on the star’s surface, which then scatter them in all directions, resulting in a darker line of sight. A set of characteristic dark lines indicates the presence of a chemical element.

Still a century later, astronomers, in the spectra of stars belonging to distant galaxies, noticed that these sets of dark lines all, on average, had a shift toward long wavelengths compared to what we observe in the laboratory, therefore a shift “toward red.”

They interpreted these shifts as a light Doppler effect, a phenomenon that occurs when a wave (acoustic or light) is emitted by a moving source relative to a receiver.

The perceived wavelength shifts toward short wavelengths as the source approaches the receiver and toward long wavelengths as it moves away from it. The power increases as the speed of the emitting source increases. We can observe this phenomenon when an ambulance passes in front of us where the siren is higher or lower depending on whether the ambulance is approaching or moving away from us. These shifts “toward the red” therefore indicated that the emitting stars belonged to galaxies moving away from ours. It was still necessary to determine whether these displacements were correlated to the distances between the emitting sources. It was not until the beginning of XXe century that astronomers had the tool to measure these distances.

Distance measurement

For stars a few light years away, the orbital parallax method is used. If we look at a star six months apart, its position changes in relation to the background of the sky. We call the parallax the angle at which we see the Earth-Sun distance from the star. This angle is equal to half the change in line of sight to the star at six-month intervals.

Diagram showing the parallax of a star
Determination of a star’s parallax.
Jacques Treiner, Provided by the author

However, this method is not suitable for distant stars or galaxies because the parallax is too small to be measured and the distance between the Earth and the Sun is relatively too small.

The solution was found in 1908 at Harvard, where a young astronomer, Henrietta Swan Leavitt, measured the brightness of stars belonging to a nebula visible in the southern hemisphere, the Little Magellanic Cloud (M). At the turn of the century, advances in instrumentation – telescopes and photography – made it possible to compile the first major catalogs of stars.

At Harvard, images taken by astronomers (mostly men) were analyzed by a team of a dozen women, and Henrietta Leavitt was interested in variable stars, the so-called Cepheids, because the first was discovered (in 1784) in the constellation Cepheus. These are giant stars whose brightness varies with a periodicity ranging from the order of a day to a few months.

Leavitt discovered a relationship between a star’s period and its brightness. The brighter it is, the greater the period. Since they all belong to the same group of stars, they can all be considered to have approximately the same distance from Earth, d (M), so that the differences in brightness reflect their differences in inherent brightness.

Then imagine that we see a Cepheid in another galaxy. We measure its period P and compare it with the Cepheids in the Magellanic Cloud. This makes it possible to determine the brightness L (M) that it would have if it were at the distance d (M). However, the apparent brightness Lap decreases as the square of the distance: Lap = L (M) 〖d (M)〗2/ d2. When we know the distance to the Magellanic cloud, we derive the distance d of the Cepheid.

We can also calibrate the period-distance ratio by measuring the period of cepheids in our galaxy, the distance of which we know by parallax measurement, and use it to determine the distance from the small Magellanic cloud.

In any case, there was the desired tool. From the measurement of the period of a Cepheid one could deduce its distance.

The universe is expanding

In the beginning of XXe century, the question of whether all visible celestial bodies belong to our galaxy, or whether there are other galaxies separate from ours, was discussed. It was the measurement of the distances described above that decided the debate, the Milky Way became, among others, a galaxy.

But it is also the method that enabled American astronomer Edwin Hubble to highlight the expansion of the universe. He noted that there was a correlation between the speed at which a galaxy moves away and its distance. The farther away a galaxy is, the greater its speed of removal.

This expansion is characterized by “Hubble’s constant H0”, which indicates how much the speed increases when the distance increases by one million parsecs (Mpc), a distance corresponding to 3.2 million AL. At the moment, when moving away from a megaparsec, the speed of celestial bodies increases by 74 km / s.

Immediate consequence: If we go back in time, the universe contracts, its density increases. How far ? Good question, but that’s another topic, Big-Bang!

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