Ask an expert: Astronomer

Paola Caselli is Professor of Astronomy at the University of Leeds. She was a resident expert on the site and users had the chance to ask her their questions about the universe.

Why is the universe expanding? How did scientists find out that it is expanding?

The expansion of the universe is due to the initial event that set the universe in motion: the Big Bang. Remember that it is space itself that expands (according to Einsteins's general theory of relativity), i.e. galaxies do not travel through space, space expands around them.

The expansion of the universe was discovered in 1929 by Edwin Hubble, who found that the optical spectra of galaxies were "redshifted" (shifted at lower frequencies) proportionally to the galaxy's distance. This means that when you look at the light coming from galaxies, some of it will appear redder becaus eit is moving away.

Hubble's law

Considering the relation between shift in frequency and velocity (i.e. the Doppler effect), this finding implied that the speed at which a galaxy moves away must be proportional to its current distance. This behaviour is similar to that produced by an explosion: bits of shrapnel are given a wide range of speeds, and those that are moving fastest travel the largest distance in a given amount of time. In summary, the implication of Hubble's law is that the universe is expanding!

If the universe is always expanding, what is it expanding into?

To answer this question one needs to remember that the actual universe includes all space; there is nothing beyond it, because there is no "beyond". In the book "Universe" by Freedman and Kaufmann III there is a sentence that puts it really clearly: "asking 'What lies beyond the universe?' is as meaningless as asking 'Where on the Earth is north of the North Pole?'"

What do you think is beyond the edges of the universe? Do you think we can ever travel (or see) that far?

This question is similar to the previous one and could be answered in a similar way. Basically, the universe itself always has and always will consist of all space. The expansion of the universe is the expansion of space and the universe does not have an edge.

There are however theories that consider the possibility of a multiverse. The multiverse consists of many (perhaps infinitely many) universes. The concept of another universe is difficult to define. It would be a disjoint region that occupies dimensions separate from (and physically inaccessible to) those of our universe - meaning we could never travel there conventionally.

This other universe might, however, be connected with ours via a wormhole (a passage between two black holes - have a look at "The Cosmos" by Pasachoff and Filippenko). A way to produce other universes would be through distinct quantum fluctuations and for this we need a fully self-consistent quantum theory of gravity that unites relativity with quantum physics. Unfortunately, this as well as others hypothesis are currently untestable so no definitive conclusions can be drawn.

Final point: there is also an intrinsic limit beyond which we cannot "see" anything, and this is the size of the observable universe (~10^10 light years), i.e. the distance that the light can travel (limited by its finite speed) since the beginning of the Universe 13.7 Gyr* ago.

• Gyr is the shorthand for 'billion years'. So 13.7 Gyr is 13.7 billion years

How long do you think life has been around in the universe? Could there be aliens living near us and if so could we see them in our lifetime?

So far, we only know of one place in the Universe where life is present: our Earth. It is then difficult to extrapolate our (limited) knowledge to the rest of the Universe. We can however put together some numbers and try to give a qualitative answer to your good question:

Our Earth formed 4.6 Gyr ago. The first fossil record of a living microorganism (already quite advanced!) is dated 3.5 Gyr, so this means that life originated on Earth soon after its formation. It was likely to be soon after the period of heavy bombardment by interplanetary rocks, such as large meteorites and comets, which lasted about 700 Myr. Unfortunately it is not possible to find this out because of the active tectonic activity of the Earth, which deleted any record of the first billion years.

Life could take 1 billion years

Considering that the Universe is 13.7 Gyr old, this means that from what we know we can set a limit to the development of life in the universe of about 10 Gyr. However, the first stars in the universe appeared only 400 million years after the Big Bang (i.e. 13.3 Gyr ago) and the first galaxies soon after that (13.2 Gyr ago). If the interstellar medium was soon enriched with heavy elements (formed inside stars) via supernovae explosions (as it appears from recent observations of high redshift quasars), then planets were able to form and organic molecules could assemble. Therefore, if also on other planets life takes about 1 billion years to originate, we can (very) tentatively say that life may have appeared in the Universe as early as 12.2 Gyr ago.

Exploring Gliese 581

There are about 100 billion stars in our Galaxy and there are at least 100 billion galaxies in our Universe. A star like our Sun has a life time of about 10 Gyr, but the majority of stars have masses slightly smaller than our Sun (the so-called M-stars) and can survive for more than 200 Gyr (so stars almost as old as the Universe are still all around us!). Although one needs to have a planet at the right distance from the star, so not all planets will be able to host life, I still expect life to have originated in many systems since the beginning of the universe and likely evolved more than us in some of them! It will be interesting to investigate in more detail a planet with a mass only a few times larger than our Earth which has been recently discovered around a 10 Gyr old M-star (see Udry et al. 2007, Astronomy & Astrophysics, vol. 469, pg. 43). This star (Gliese 581) is only 20 light years away from us...

How can you work out the composition of something that is so far away - like dark clouds?

We need powerful telescopes, which can collect as many photons as possible to allow us to make quantitative analyses. Radio and millimeter telescopes used for detecting molecular emission in dark clouds have huge collecting areas. The largest radiotelescope on Earth is the Arecibo Observatory, which has a diameter of 305 meters!

I'm majoring in physics and expecting my BSc soon. I'm torn between astronomy and theoretical physics and I don't know which I'd be better at. Could you tell me a little more about both?

If you're interested in both, I suggest that you think about theoretical astrophysics. Many new instruments are going to be built in the near future and we need more and more theoreticians who can help observers to interpret data.

For example, the Atacama Large Millimeter/submillimeter Array (ALMA) will give astronomers the opportunity to observe with unprecedented sensitivity and angular resolution. They will be able to see protoplanetary discs (where planets like our Earth will form) and highly redshifted galaxies at the edge of the observable universe. Our views of star and planet formation and cosmology are likely to be revolutionized, so we need more ideas and good theoreticians!

Do you think women find it hard to progress in physics? I've heard that even though more and more women are studying physics at university many of these leave the profession before they reach 30. Why do you think this is?

It is true that there are still too few women in physics and astrophysics, but it is getting better. This is a real full time job, which often requires you to sacrifice some valuable time with friends and family. I have to say that it has not been easy for me, especially because not all the people around me understood how important for me it was to do what I was doing.

"Struggle to be happy"

This becomes particularly difficult when we have children (you mention 30 years old as a critical age, and in fact this may coincide with the time when women most feel the need to have a family).

My daughter was born when I was 34 and of course my research slowed down a bit at that time. However, I remember working from home and itching to go back to full time work (although of course I was also enjoying playing with my little one!). Looking back now (my daughter is almost 9 years old) I can see that I should have probably spent more time with her, but I tried hard to have as much quality time with her as much as possible and (as far as I can see) she is a happy child.

In summary, I strongly suggest that you don't give up dreams in difficult times and always do your best. We should all try to make to make others and ourselves as happy as possible. "Struggle to be happy" is my motto.

Your shout!

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