How do we know that our universe is expanding?

Let’s start with a small experiment that will give us an image of an “expanding universe”. This universe will be an inflatable ball.

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 is gradually inflated.

As the circle grows, the distance to the center grows, as does the distance between two points on the circle. This applies regardless of the chosen starting point. To have an image of an expanding universe, it is enough to generalize the case of a surface to the case of a volume. Each point can “see” other points moving from it as if it were the center of expansion.

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

Expand on a large scale, but not necessarily locally

Now we need to explain how scientists have come to this conclusion about the visible universe, and not just an inflatable balloon.

For this, we need to observe the universe on a large scale. Neither the Moon nor the Sun moves away from the Earth, nor do the other objects in the solar system. The stars of our galaxy, the Milky Way, are not moving away from us. And even the Andromeda galaxy, located more than two million light-years (AL) 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 a scale 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 close locally, and even colliding.

Example of colliding galaxies: the Mouse galaxy, located 301 million AL from our galaxy
Example of colliding galaxies: 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 (Americans, in this case) observed that distant objects in the sky are moving away from us, and that their speed of removal is much faster. you are far To do this, we need to measure, for each object, its distance from us and its speed.

Speed ​​measurement

The turning point came when physicists examined the light coming from the stars, starting with the Sun. Newton understood that white light is composed of a continuum of wavelengths, but in the early 19th century Frauenhoffer, a German physicist, noted the presence of dark lines in the solar spectrum.

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

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

A century ago, astronomers noticed, in the spectra of stars belonging to distant galaxies, that these sets of dark lines were all, in general, a shift to long wavelengths compared to what we observe in the laboratory, therefore a shift “toward the 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 observed wavelength shifts to short wavelengths when the source approaches the receiver and to long wavelengths when it moves away from it. The effect increases as the velocity of the emitting source increases. We can observe this phenomenon when an ambulance passes in front of us, the siren is higher or lower depending on whether the ambulance is approaching or moving away from us.

These “redshift” shifts therefore indicate that the emitting stars belong to galaxies moving away from us. It remains to be determined whether these offsets are consistent with the distances to the emission sources. It was not until the beginning of the 20th century that astronomers had tools to measure these distances.

Distance measurement

For stars several light-years away, the orbital parallax method is used. If we look at a star that is six months away, its position relative to the background of the sky will change. We call parallax the angle at which we see the Earth-Sun distance from the star. This angle is equal to half the change in the line of sight of the star 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 measure, the Earth-Sun distance is relatively 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, photographs taken by astronomers (mostly men) were analyzed by a group of a dozen women, and Henrietta Leavitt became interested in variable stars, the Cepheids, so called because they were the first to be discovered (in 1784) in the constellation Cepheus. These are giant stars whose brightness varies with a periodicity from the order of a day to several months.

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

Now imagine that we found a Cepheid in another galaxy. We measure its phase P and compare it to the Cepheids in the Magellanic Cloud. This makes it possible to determine the light L (M) it will have if it is at a distance d (M). However, the apparent light Lap decreases as the square of the distance: Lap = L (M)〖d (M)〗2/d2. Knowing the distance to the Magellanic Cloud, we can determine the distance d to Cepheid.

We can also calibrate the time-distance relationship by measuring the phase of the Cepheids in our galaxy, whose distance we know by measuring parallax, and use this to determine the distance from Small Magellanic Cloud.

In any case, there is a desired tool. From measuring the period of a Cepheid, one can infer its distance.

The universe is expanding

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 apart from ours was debated. It is the measurement of the distances described above that settled the debate, the Milky Way became a galaxy and so on.

But it is also the way that allowed the American astronomer Edwin Hubble to promote the expansion of the universe. He noted that there is a correlation between the speed at which a galaxy moves and its distance. The more distant a galaxy is, the greater its rate of removal.

This expansion is characterized by the “Hubble constant H0”, which shows how much the speed increases when the distance increases by one million parsecs (Mpc), a distance equal to 3.2 million AL. Currently, when one moves one megaparsec away, the speed of celestial objects increases by 74 km/s.

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

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

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