How do we know that our universe is expanding?

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

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

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

Seen from an arbitrary point on the surface, all other points move as if it were the center of expansion – Jack Traynor (via Conversation)

Expansion on a large scale, but not necessarily local

Now we need to explain how scientists came to this conclusion about the observable universe, not just an inflated balloon.

For this we need to observe the universe on a large scale. Neither the Moon nor the Sun move away from Earth, nor do other objects in the Solar System. The stars in our galaxy, the Milky Way, are not moving away from us. Even the Andromeda Galaxy, which is two million light-years (AL) away, is not moving away from us. On the contrary, it is approaching us at a speed of 500 kilometers per second.

Is the universe really expanding? Yes, but on a scale of tens, millions and billions of AL. On average, galaxies move away from each other, but that doesn’t prevent some from locally coming closer and even colliding.

Example of colliding galaxies: 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’ve known about the expansion of the universe since the 1920s, when astronomers (Americans, in this case) noticed that distant celestial bodies were moving away from us, and that their speed of removal was greater. To do this, we need 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 the stars, starting with the Sun. Newton realized that white light is composed of a continuum of wavelengths, but in the early 19th century Frauenhofer, a German physicist, noticed the appearance of dark lines in the solar spectrum.

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

Dark lines in the continuous solar spectrum
Dark Lines in the Continuous Solar Spectrum – Jack Trainor (in conversation)

Still a century later, astronomers noticed, in the spectra of stars belonging to distant galaxies, the average of all sets of these dark lines, a shift towards longer wavelengths than what we observe in the laboratory, hence a shift “towards” red”.

They explained these changes as the light Doppler effect, a phenomenon that occurs when a wave (sound or light) is emitted by a moving source relative to a receiver.

The perceived wavelength shifts toward shorter wavelengths as the source approaches the receiver and toward longer wavelengths when moving away from it. The effect increases as the speed of the emission source increases. We may notice this phenomenon when an ambulance passes in front of us, the siren getting higher or lower depending on whether the ambulance is approaching or moving away from us.

These shifts “toward the red” therefore indicate that the ejected stars belong to galaxies moving away from us. It was still necessary to determine whether these offsets were related to the distance to the emission sources. Astronomers did not have tools to measure this distance until the early 20th century.

Distance measurement

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

Determining the parallax of a star
Determining the Parallax of a Star – Jack Trainor (in 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 at Harvard in 1908, where a young astronomer, Henrietta Swan Levitt, measured the brightness of stars belonging to the Small Magellanic Cloud (M), a nebula visible in the southern hemisphere. 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, photographs taken by astronomers (mainly men) were analyzed by a team of a dozen women, and Henrietta Leavitt was interested in variable stars, the Cepheids, so called because the first were discovered in the constellation Cepheus (in 1784). These are giant stars whose brightness varies with periodicity from the order of a day to several months.

Leavitt discovered a relationship between a star’s period and its brightness. The brighter it is, the longer its duration. 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 differences in their intrinsic luminosity.

Imagine then that we see a Cepheid in another galaxy. We measure its period p and compare it to Cepheids in the Magellanic Cloud. This makes it possible to determine its luminosity L (M) when it is at a distance d (M). However, the apparent illuminated lap decreases as the square of the distance: lap = L (M)〖d (M)〗2/d2. Knowing the distance to the Magellanic Cloud, we find the distance d to Cepheid.

We can calibrate the period-distance relationship by measuring the periods of Cepheids in our galaxy, whose distances we know by parallax measurements, and use that to determine the distance to the Small Magellanic Cloud.

In any case, the desired tool was. From measurements of the Cepheid’s period, one can determine its distance.

The universe is expanding

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

But it was the method that allowed the American astronomer Edwin Hubble to highlight the expansion of the universe. He observed that there is a relationship between the speed at which a galaxy moves and its distance. The more distant a galaxy is, the faster it is moving away.

This expansion is characterized by the “Hubble constant H0”, which indicates how much the speed increases when the distance increases by 1 million parsecs (Mpc), equivalent to 3.2 million AL. Currently, when one moves away from one megaparsec, the speed of the celestial object increases by 74 km/s.

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

The analysis was written by theoretical physicist Jacques Trainor of Paris City University.
The original article was published on the site the conversation.

Leave a Comment