Q & A

Many thanks for the book, which I have enjoyed very much.

At a number of points the concept of "collapse" or "gravitational collapse" is important - for example pp. 52 and 66. Although I understood the general point being made, I wanted to know a little
more about what actually happens when there is such a collapse - or perhaps I mean what happens during the collapse. I particularly wanted to understand in more detail the comment on page 66 about gas clouds collapsing due to the shock wave from a dying first generation
star. I would be grateful for any further explanation that might help me "see" what actually goes on at these points.

Judith-Anne MacKenzie- Wimbledon, London

Your question made me think, which is always a good sign! Perhaps the first thing to do is to show you a simulation of the collapse we're talking about

This [redshift z=18.3 picture] is a picture of a region of the Universe as it was something like 210
million years after the Big Bang. The lighter the colour, the denser the region of the Universe represented by that pixel is.

We can run the Universe forward in the computer, and here's what the result is a billion
[z=5.7 picture],

and almost five billion years [z=1.4] after the Big Bang.

By the time we've reached today [z=0] we've formed a massive supercluster.

This is the kind of collapse we're talking about, material gathering in relatively dense regions under the influence of gravity. Why does it happen? Regions which are already slightly more dense - which have more stuff - than the average will have a slightly stronger gravitational pull on their surroundings, and so will pull in more material, accelerating the process.

Although I've used the example of the formation of a supercluster, the same essential physics governs the formation of galaxies and - on a much smaller scale - stars and perhaps planets.

[photos from: http://www.mpa-garching.mpg.de/galform/virgo/millennium/ ]

Chris Lintott

Is it true to say that the universe is as big as the light spewed out by the big bang (given that light is the fastest thing we know of)or do astronomers define the size of the universe in terms of what is 'observable' - is it possible that there is something beyond observable light?

Scott Adcock - Carshalton, Surrey, England

We try to divide the 'observable Universe' - which is indeed as large as the region from which light has had time to reach us since the Big Bang - from the Universe itself. If the theory of inflation is
correct, then the Universe must be many, many times the size of the observable Universe so that we see only a fraction of the whole.

Chris Lintott

1) I like the thought of black holes not being exempt from the nothing lasts forever scenario. I see the logic of proton decay, but not the virtual particle bit. You put forward a scenario of a particle and an antiparticle appearing, and before they can annihilate each other one is sucked over the event horizon, the other ejected. How is this tantamount to the black hole losing mass? Doesn't it gain the mass of the one it sucks in? Or are we assuming that it only loses mass if it is the antiparticle that gets sucked in and it annihilates one of the black hole's particles? Or were the pair formed out of a vaccuum that counted as part of the Black Hole's mass?

2) Observing a sixth form Physics lesson today I was suddenly reminded that, while space is expanding, galaxies themselves are not. There was a picture of a balloon marked with little circles, then blown up - the circles also expanded. Is it gravitational force working against the expansion of space that prevents the galaxies from expanding along with the rest of the Universe? Or, could it have anything to do with their unusual (uniform speed) rotation? Or do we not know?

Thanks very much - by the way I can't resist telling you I wrote a review on Amazon of BANG! which I hope will encourage even more people to buy it! I'll send it to you if you like. Best physics teacher I've had in about 14 years!

Alice Sheppard - Pembrokeshire, Wales, UK

Hi Alice

Good questions (and an excellent review, even if I do say so myself!).

1. The explanation of the black hole losing mass won't satisfy you - very little in the quantum realm makes much sense! - but let me try. We know, as we can tell from experiments, that particles and antiparticles appear and disappear all the time. The energy for creating them is in some sense `borrowed' from the Universe, and is returned when the particles collide.

Having a black hole around ruins this neat arrangement. The particles appear, but cannot
recombine because one of them has disappeared into the black hole. Yet the Universe has lent its energy, and it must be paid back - and the only place for that energy to come from is from the black hole. Viewed by a distant observer, it looks like the black hole has emitted a particle and must have lost the corresponding amount of mass.

I know this all sounds very contrived, but this is how the Universe seems to work. The fanciful description involving `borrowing energy' from the Universe is not a million miles from the kind of description we might right about reactions involving sub-atomic particles. Of course, we have yet to actually detect the (very faint) Hawking radiation, so it may be completely wrong.

2. You're right - it is the gravitational force working against the expansion of space that prevents
the galaxies from expanding. Relative to the rest of space, a galaxy is extremely dense and so - locally - there is enough mass for gravity to overcome the expansion and hold it together. In fact, larger objects can still be held together; our Local Group of galaxies is resisting the expansion, which is why the Andromeda galaxy is approaching our own.

Chris Lintott

I was just wondering how many years did you spend travelling to Tenerife Island ? Thanks in advance for your time and reply - can't wait to open the new book!!

Andrew G. Reid - St. John's, Newfoundland, Canada

About 3 years initially.. but I've been going back ever since. It's close to Heaven up there! (but don't go to Playa Las Americas!!!)

Cheers

Brian May

Hello Brian, i'm a big fan.? I'm only thirteen, but I've read a book from Patrick Moore, "The Atlas of The Universe", and I've seen pictures of the sun.? I saw one of the sunspots, and it kept moving diagonally to the right. Does this mean the sun rotates?

Alex Mullins - Phoenix, Arizona

Dear Alex,

I'm very happy to hear you are already into Astronomy at the age of thirteen ... I was too, and with Sir Patrick Moore's books, you are in safe hands ! You know, we have a lot in common ... I did exactly what I think you are doing ... I projected the image of the Sun onto a piece of white card every day for a couple of months (taking care NEVER to look through the Sun through any kind of telescope, or binoculars) and drew the sunspots as I saw them on a succession of circles drawn on a large sheet of paper. I discovered quickly that they were changing a lot in shape from day to day, but also moving across the face of the Sun. Some big ones even made it around the other side, to transit again across the visible face of our yellow star. Later I discovered that the axis of rotation appeared to change too ... as the Earth journeys around the Sun through the year.

Yes, the Sun does rotate - in about 26 days (not far off a lunar month, strangely), but it actually doesn't rotate as a solid body. It's a huge ball of gas, and the rotation period is different at its equator from nearer its poles. The rotation period also appears different because we are standing on the Earth, which is in orbit around the Sun ... we are not travelling at a uniform speed either ... because the Earth's orbit is elliptical, not circular ... so the Sun appears to vary in its rate of spin. I'm sure all this makes perfect sense to you ... it's fun to visualise all this stuff.

There's an interesting account at this URL:
http://brahms.phy.vanderbilt.edu/a103/labs/web_propsun.shtml
They show these two pictures of the Sun on successive days.

I couldn't resist putting them together next to each other like this, because they make a nice stereo pair ... which can be viewed in 3-D ! (see my 'Soapbox' on BrianMay.com for some other stereo's ! ) It's plain to see these groups of sunspots drifting across, as the Sun rotated.

The images at the bottom of the page are even more amazing ....from the SOHO satellite .... also making a stereo ... I must try to find the original large images - they would make a wonderful stereo But I digress!

Thanks for the message, Alex, and I hope Astronomy takes you to great places ...

cheers!

Brian May

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