Comment sait-on que l’univers est en expansion ?

Comment sait-on que l’univers est en expansion ?

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, marking two points on the circle. The balloon gradually inflates.

As the circle grows, the distance to the center grows, 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 suffices to generalize the case of a surface to the case of a volume. Each point “sees” that the other points move away from it as if it were the center of the expansion.

Viewed from an arbitrary point on the surface, all other points recede as if it were the center of expansion – Jacques Treiner (via The Conversation)

​Large-scale expansion, but not necessarily local

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

To do this, we must observe the universe on a large scale. Neither the Moon nor the Sun move away from the Earth, nor do other objects in the solar system. The stars of our galaxy, the Milky Way, do not move away from us. And even the Andromeda galaxy, which is more than two million light-years (LA) away, is not far 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 this does not prevent some from getting closer locally, and even colliding.

Example of a galaxy collision: the Mouse galaxy, located 301 million AL from our galaxy
Example of galaxy collision: the Mouse 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 (American, in this case) observed that distant celestial objects were moving away from us, and that their speed of departure 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 speed.

speed measurement

The turning point came when physicists analyzed the light from stars, beginning with the Sun. Newton understood that white light was made up of a continuum of wavelengths, but it was not until the early 19th century that Frauenhoffer, a German physicist, noted the presence of dark lines in the solar spectrum.

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

Dark lines in a continuous solar spectrum
Dark lines in a continuous solar spectrum – Jacques Treiner (via The Conversation)

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

They interpreted these changes as a Doppler effect of light, a phenomenon that occurs when a moving source emits a wave (acoustic or light) relative to a receiver.

The perceived wavelength shifts towards short wavelengths as the source approaches the receiver and towards long wavelengths as it moves away from the receiver. The effect increases as the speed of the emitting source increases. We can observe this phenomenon when an ambulance passes in front of us, the siren being higher or lower depending on whether the ambulance is approaching or moving away from us.

These “redshifts” therefore indicated that the emitting stars belonged to galaxies moving away from our own. It was still necessary to determine if these compensations were correlated with the distances of the emitting sources. It wasn’t until the early 20th 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 relative to the background of the sky changes. We call parallax the angle under which we see the Earth-Sun distance of the star. This angle is equal to half the change in the star’s line of sight in six-month intervals.

Determination of the parallax of a star
Determining the parallax of a star – Jacques Treiner (via The Conversation)

But this method is not suitable for distant stars or galaxies, because the parallax is too small to be measured, the Earth-Sun distance being 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 Small Magellanic Cloud (M). At the turn of the century, advances in instrumentation – telescopes and photography – made it possible to compile the first large catalogs of stars.

At Harvard, photos taken by astronomers (mainly men) were analyzed by a team of a dozen women, and Henrietta Leavitt became interested in variable stars, the Cepheids, so called because the first was discovered (in 1784) in the constellation of 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 the period of a star and its luminosity. The brighter it is, the longer its period. Since they all belong to the same group of stars, they can all be considered to be approximately the same distance from Earth, d(M), so that differences in luminosity reflect their differences in intrinsic brightness.

Let us imagine then that we spot a Cepheid in another galaxy. We measure its period P and compare it with those of the Cepheids of the Magellanic Cloud. This allows to determine the luminosity L (M) that it would have if it were at the distance d (M). However, the apparent luminosity Lap decreases with the square of the distance: Lap = L (M)〖d (M)〗2/d2. Knowing the distance from the Magellanic Cloud, we deduce the distance d from the Cepheid.

We can also calibrate the period-distance relationship by measuring the period of the Cepheids in our galaxy, whose distance we know from parallax measurement, and use this 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, its distance could be deduced.

the universe expands

At the beginning of the 20th century, the question of whether all visible celestial objects belong to our galaxy or whether there are other galaxies separate from ours was debated. It was the measurement of distances described above that settled the debate, the Milky Way became one galaxy among others.

But it is also the method that allowed the American astronomer Edwin Hubble to highlight the expansion of the universe. He realized that there was a correlation between the speed at which a galaxy is receding and its distance. The more distant a galaxy is, the faster its removal rate.

This expansion is characterized by the “Hubble constant H0”, which indicates how much the speed increases when the distance increases by one million parsecs (Mpc), a distance equivalent to 3.2 million AL. Currently, when one moves away from a megaparsec, the speed of celestial objects 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, the Big-Bang!

This analysis was written by Jacques Treiner, a theoretical physicist at the University of Paris Cité.
The original article was published on the site of The conversation.

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