The cosmos is vast, so how do we measure it?

One of the major challenges to communicate on space and space-time is that the universe is so large that the scales are very difficult to imagine. Even trying to have an idea of ​​our solar system can be difficult. If I had to make a model on the scale in which the sun measures 1 centimeter, I should store Pluto at 42 meters. I don’t know about your home, but mine does not have a ballroom – which would concern this length.

However, our solar system is tiny on the ladder of the Milky Way. Ignoring the fact that our galaxy exists in a halo of invisible dark matter which extends far beyond the visible parts, the Milky Way is so large that it would take around 100,000 years to cross it. On the other hand, the light passes from the sun to Pluto in just 5.5 hours.

You may have noticed that I have gone daily distances to the units linked to the speed of light. One hundred thousand light years old is 9.46 × 10²⁰ meters. How could I even tell you to visualize this? I could as well say that these are Bajillion ball rooms. And the Milky Way is quite small compared to the cosmos. It is not even a particularly large galaxy. Our Andromeda neighbor is twice as large.

In addition to that, space-time develops. This does not affect distance measurements in regions linked to gravitation such as our solar system or the Milky Way. This does not even necessarily affect the distances between the galaxies: the Milky and Andromeda path in fact go to each other, although the possible collision will look more like a sweet dance than cars that crush, and it is at least 4.5 billion years, so do not be afraid!

But on the biggest scales, space-time stretches and groups of galaxies are far from each other. This is known as Hubble’s expansion, and it means that many distance measures in the universe will change. In billions of years, people will get a different figure for the gap between us and the Virgo Galaxy cluster, which is currently 50 million light years.

In principle, these figures are impressive, but it is also understandable that they invite some skepticism. First of all, how can we be if some of these measures? This is actually a subtle problem in astronomy. The way we do it is to build a “scale” of measurements, often using known brightness objects as certain types of stars, which allow us to assess the distance.

The lowest – the easiest – is to use Variable Stars Cepheid, which drive regularly, to calculate how far things are. These are effective up to a certain distance, how much we have to move on. Over the past 30 years, astronomers have used specific types of supernovae or dying stars, because we know how to characterize the way their light is extended by the expansion of space-time. There are also other ways, some using what we know about the brightest red giant stars.

We have a high level of confidence in our ability to measure long distances, but I understand why, despite this, I received some questions from readers related to this. One concerns what happens to light as the universe develops. A standard part of our cosmological image is that, just like the frequency of a mermaid that moves away from us is offset, light waves extend as space-time develops, redly red. The measurement of this delay to red is crucial for our use of supernovae to assess the distance, as mentioned above.

Redshift also means that light is lower than before. But there is no apparent place that the lost energy goes, which seems suspicious. Usually, when we get rid of energy, it goes somewhere. This is necessary in Newtonian physics. However, it is not in general relativity. In other words, the thing that allows us to measure long distances is also something that violates our daily concepts on the way energy moves into the universe.

Another related question that recently came from a reader concerns photos of distant galaxies, such as those among the first images of the New Vera C. Rubin Observatory. Shouldn’t we see galaxies as a blur, when space-time develops?

The important thing to keep in mind here is that “seeing” the expansion of space-time is not like watching Lewis Hamilton in F1. It’s much more like watching F1 if a race has taken billions of years, really, really far. On this scale, cars would not visibly move. The only way we know that the galaxies are moving away from us is to measure something like the gap towards red, and this is only a measure of the way in which light is stretched, and not an observation of the real -time movement of the galaxy.

I particularly like these kinds of questions because they go to the heart of the metaphors that we use scientific communicators to speak to our audience. I appreciate that New scientist Readers push these metaphors to their limits!

Chanda week

What I read

For reasons that will become public, many Alice’s Adventures In Wonderland.

What I look at

I finally saw and appreciated Station eleven.

What do I work

I thought a lot about what the quantum fields are really. Odd!

Chanda Prescod-Weinstein is an associate professor of physics and astronomy, and basic member of the faculty in women’s studies at the University of New Hampshire. His most recent book is The Disorderred Cosmos: A Journey Into Dark Matter, Spacetime and Dr.