Hello

So i have not posted on this forum for a long time, but there are two things that i have been wondering about (among others) and i wondered if anyone on this forum could provide clarification...

Firstly, if the universe is expanding in all directions and if we look out with a telescope in all directions we see the older presumably smaller universe, does that mean that the larger universe now is contained within the smaller universe then?..

and secondly, this is about radiation, if radioactive material becomes less radioactive with time, does that mean it was more radioactive in the past, even in the quite recent past (during the history of life on earth)?

i appreciate that perhaps these are not very interesting questions but well maybe you can help....

Rob

The universe is the universe is the universe. We don't look out and see a different universe. The further we look ... e.g. the older the light....the further back in time it is....but it is still the same universe.

Radioactive decay has various types much about this on wikipedia: http://en.wikipedia.org/wiki/Radioactive_decay

It's not that it was 'more' radioactive in the past, as far as I know, it depends on the type of decay and the decay products, etc.

kennyc wrote:The universe is the universe is the universe. We don't look out and see a different universe. The further we look ... e.g. the older the light....the further back in time it is....but it is still the same universe.

But it was, indeed, a smaller universe at the time that the light was emitted. Considerable universal expansion may occur between emission and detection, however, so it does not imply that a quart is being fitted into a pint pot, so to speak!

Gentleheart wrote:Hello

So i have not posted on this forum for a long time, but there are two things that i have been wondering about (among others) and i wondered if anyone on this forum could provide clarification...

Firstly, if the universe is expanding in all directions and if we look out with a telescope in all directions we see the older presumably smaller universe, does that mean that the larger universe now is contained within the smaller universe then?..

and secondly, this is about radiation, if radioactive material becomes less radioactive with time, does that mean it was more radioactive in the past, even in the quite recent past (during the history of life on earth)?

i appreciate that perhaps these are not very interesting questions but well maybe you can help....

Rob

Hiya

The second question is fairly straightforward to answer: yes!

That doesn’t necessarily mean there was more of that particular kind of radioactivity around in the past.

A substance is radioactive because it is unstable. Carbon 14, for instance, has too many neutrons in its nucleus to be stable: it has six protons like all carbon nuclei, but eight neutrons as opposed to the normal six. When it decays, one of the neutrons decays into a proton, and an electron and an antineutrino are emitted, and the result is a nitrogen nucleus with seven protons and seven neutrons, which is stable. So a given sample of carbon 14 will, over time, decay into more and more nitrogen, so as far as that goes there is less and less carbon 14 around. However, the stock of carbon 14 on Earth is constantly being replenished by the effect of cosmic rays, which converts a nitrogen nucleus into a carbon 14 one.

Uranium, on the other hand, another unstable nucleus, is not created on Earth. The stock of uranium on our planet is derived from supernovae in the remote past which seeded the dust, which later collapsed to form our planet, with heavy elements. Due to the decay of uranium there was indeed more of this kind of radioactivity on (and in) Earth in the past than there is now.

Your first question I find a little hard to understand. What we see when we look into the deep sky is the universe as it was in the past, because the light that we are registering has taken all that time to travel to us from where it was originally emitted. Now, the universe is bigger than the one we can see; but we can’t see it yet, because light hasn’t yet had time to reach us. In fact the remoter parts of the universe we’ll never be able to see, because the expansion rate is faster than the speed of light: as the light attempts to travel towards us, it is being swept away from us by the expansion of the universe faster than its own speed, so that from our point of view it is travelling backwards.

Not sure whether that in any way addresses your question.

haha i understand, thank you

The answer to the first question is I think no, All the matter and energy that is in our universe now was contained in a smaller region in the past, but for the universe to be contained in itself? I don't think that makes sense. What happens is that light emitted far away travels a curved path in spacetime to reach us. Essentially you can think of it is that by the time the light emitted by a distant star has traveled half way towards us the remaining distance has grown due to expansion. For very distant stars this factor becomes bigger and bigger, to a point c light years away where emitted light will never reach us.

In answer to the second the answer is probably yes, the overall radiation was probably higher in the past. However many elements have very long half lives of up to billions of years e.g. Uranium-238, only about half of which will have decayed since the Earth first formed. These long-half life elements create other elements when they decay, as part of a "decay chain", many of which are themselves radioactive. Thus for many elements with shorter half-lives the amount of that element present (and thus the radiation emitted) is in a state of almost equilibrium, with new atoms being created almost as fast as the old ones decay.

There are also other methods of creation of radioactive isotopes, one of these being the creation of Carbon-14 in the upper atmosphere at a fairly constant rate by the interaction of cosmic rays on Nitrogen atoms, again introducing a supply of new radioactive atoms at close to the rate they decay. In fact this last method is what allows radiocarbon dating to work - since living animals breathe in carbon-14 atoms from the atmosphere where they exist at fairly constant levels, but stop taking them in when they die, meaning the amount of atoms can be fairly accurately known at the time of death, thus allowing a calculation based on the decay rate to estimate when an animal died (so long as it was within the last 50,000-60,000 years or so).

Edit: Beaten to the punch, and more eloquently put as well. Curse you evolving!

Beat you, Thommo!

Thommo wrote:The answer to the first question is I think no, All the matter and energy that is in our universe now was contained in a smaller region in the past, but for the universe to be contained in itself? I don't think that makes sense.

Indeed, it would not make sense, but it was not what he meant, I think.

Evolving wrote:Beat you, Thommo!

Now you're just rubbing it in. I'm off to take a bath to calm myself down.

With bubbles and scented candles and maybe a nice glass of Chablis.

Very wise.

Meh, you're both losers, I was first in.

kennyc wrote:Meh, you're both losers, I was first in.

But with one incomplete answer.

Kenny wins the internet.

Evolving, thank you too, er i think i thought that we would see light from the very begining of the universe in every direction we looked, that we were surrounded by a singularity, assuming we had a telescope that could see that far and the heat of the universe didn't prevent us from seeing before 300,000 years ago (according to Lawrence Krauss on youtube anyway), and of course that might not make sense, although that doesn't mean it couldn't be so...the universe would just be weirder than we thought. ok, i'll leave it there thanks again.

thanks Thommo, appreciated nevertheless!

Evolving wrote:Kenny wins the internet.

Woo Hoo and with my metafisics diploma from SOS I would be king!

Gentleheart wrote:...and the heat of the universe didn't prevent us from seeing before 300,000 years ago (according to Lawrence Krauss on youtube anyway)...

You may simply have mistyped here, but the barrier is not 300,000 years ago, but when the universe was 300,000 years old. That is much longer ago!

The reason why we can't see anything from before then is because the universe, when it was younger than about 300,000 years, was opaque to light: just as the Sun is, for instance. Any light (and any electromagnetic radiation) attempting to travel through the Sun is constantly being absorbed and re-emitted and scattered by the particles that it encounters. The light that we see coming from the Sun is emitted at its surface (or not far below). Similarly, the oldest electromagnetic radiation that we can measure coming from the sky was emitted from what we call the last scattering surface, i.e. when most of the universe ceased to be opaque to light. It was emitted over the whole range of possible wavelengths (as blackbody radiation), so it included visible light; but because of the expansion of the universe its wavelength has been dramatically extended and it now forms the cosmic microwave background.

yes yes, that is mistyping!! I meant 300,000 years old, thanks

sorry double post

Evolving wrote:Kenny wins the internet.

With such a crappy answer as his? You must be kidding?

David, I've got your number, drop the bullshit and stop trolling me.
It should be clear by now that I'm ignoring your attempts to provoke me.

If you don't stop I'll get the former mods with retained powers to put you away.