How do we know our universe is expanding?

How do we know our universe is expanding?

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

We mark any point on the surface with a pen, draw a small circle around it, mark 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 any two points on the circle. This applies regardless of the starting point chosen. It suffices to generalize the case of a surface to the case of a volume to obtain a picture of an expanding universe. Each point “sees” other points moving away from it as if it were a center of expansion.

Seen from an arbitrary point on the surface, all other points recede as the center of expansion.
Seen from an arbitrary point on the surface, all other points recede as if they were a center of expansion – Jack Traynor (via The Conversation)

Expansion on a large scale, but not necessarily local

Now we must explain how scientists arrived at this conclusion in terms of the observable universe, not just an inflated balloon.

For this we need to observe the universe at large. Nothing the moon No the sun No more than other objects in the solar system, do not move away from Earth. The stars of our galaxy, the Milky Way, are not moving away from us. And also Andromeda GalaxyIt is more than two million Light years away (AL) does not move away from us. Instead it is approaching us at a speed of 500 km per second.

Is the universe really expanding? Yes, but AL in tens, hundreds of millions and billions of sizes. On average, Galaxies move away from each other, but that doesn’t stop some from getting close and clashing internally.

Example of a colliding galaxy: The Mouse galaxy, located 301 million AU from our galaxy.
Example of a collision of galaxies: The Mouse Galaxy, 301 million AU 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 objects were moving away from us and that their speed of removal was even greater. To do this, for each object, we can measure its distance from us and its speed.


The breakthrough came when physicists began studying the light from the stars, starting with the Sun. Newton It was understood that white light is made up of a continuum of wavelengths, but Frauenhoffer, a German physicist in the early 19th century, did not notice dark lines in the solar spectrum.

These “missing” wavelengths are caused by absorption by elements on the star’s surface, which are then scattered in all directions, resulting in a darkening of the line of sight. 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 Traynor (via The Conversation)

A century later, astronomers observed that all these dark lines in the spectra of stars from distant galaxies shifted, on average, to longer wavelengths than what we observe in the laboratory. Red”.

They explained these contradictions as a Doppler effect Flashing, 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 as 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 will be louder or louder depending on whether the ambulance is approaching or moving away from us.

These shifts are “redward” and therefore indicate that the emitting stars belong to galaxies moving away from us. It is important to determine whether these offsets are related to the distance of the emitting sources. It was not until the early 20th century that astronomers had instruments to measure this distance.

Distance measurement

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

Determination of parallax of a star
Determining the Parallax of a Star – Jack Trainor (via The Conversation)

But this method is not suitable for distant stars or galaxies because the parallax is immeasurably small, the Earth-Sun distance being relatively small.

The solution was discovered in 1908 Harvard, 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 tools – Telescopes And Photography – made possible the creation of the first large catalogs of stars.

At Harvard, photographs taken by (mainly male) astronomers were analyzed by a group of a dozen women, and Henrietta Leavitt had been interested in the variable stars Cepheids since they were first discovered (in 1784) in the constellation Cepheus. These are giant stars whose brightness varies with time intervals from the order of a day to a few months.

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

Imagine we have a Cepheid in another galaxy. We measure its period P and compare it to the Cepheids period of the Magellanic Cloud. This helps to determine the luminosity L (M) that it has if it is at a distance d (M). However, the apparent luminance lap decreases as the square of the distance: Lap = L (M)〖d (M)〗2/d2. Knowing the distance to the Magellanic Cloud, we subtract d from the Cepheid’s distance.

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

Anyway, the desired tool was there. From the measurement of a Cepheid’s period, one can deduce its distance.

The universe is expanding

At the beginning of the 20th century, the question of whether all visible celestial bodies belonged to our own galaxy or existed in other galaxies than ours was debated. The measurement of the distances described above settled the debate, and the Milky Way became a galaxy among others.

But this is also a permissible method Astronomer American Edwin Hubble highlighted the expansion the universe. He observed that there is a relationship between the speed at which a galaxy moves away and its distance. The more distant a galaxy is, the faster its removal speed.

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

Immediate effect: If we go back in time, the universe is contracting and its density is increasing. how far Good question, but that’s another subject of Big-Bang!

The analysis was written by Jacques Treinor, a theoretical physicist at Paris City University.
The original article was published on Conversation.

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