B What does it mean to say that "space" has a temperature?

1. Nov 4, 2018

Graeme M

I was thinking about the fact that space contains all sorts of matter and that these objects have temperatures. But then I read that space itself has a temperature depending on where you are. The cosmic background radiation for example has a temperature of 2.7K. Can someone explain what that means?

I assumed that temperature is just a measurement of the thermal energy of an object, whereas space is not material. I would have thought space has no temperature at all but that objects in space can be heated by whatever thermal radiation is present.

Does radiation itself have a temperature?

2. Nov 5, 2018

Drakkith

Staff Emeritus
Generally it means that if you place an object at a location in space it will reach a certain temperature after a time. Far away from any stars, an object will eventually reach 2.7 K, whereas on the Moon in direct sunlight it will reach about 400 K. However there are some exceptions. Some areas of space have local nebulae that are well above 2.7 K, but they are so sparse than an object placed there will still be equalize to a temperature close to 2.7 K.

3. Nov 5, 2018

Bandersnatch

The 2.7K is not the temperature of space. It's the temperature of CMB radiation, which happens to be filling space.

Radiation can have temperature in the following sense:
Objects that can be approximated as black bodies radiate with a characteristic spectrum which is dependent solely on their temperature (the black-body spectrum).
So if you see a spectrum that is shaped like the black-body spectrum, you can assign temperature to this shape.
Normally, this temperature can be translated as meaning simply 'the temperature that the radiating object was at'
But, with expansion of space, the original spectrum has been stretched, so that now the radiation looks like what would be emitted by a black body at 2.7K, even though there never existed any such radiator (the gas which emitted CMB was at 3000K).

Saying that space has temperature is sometimes used as a shorthand, with the meaning that if you put an idealised thermometer in empty space, far away from any source of light other than CMBR, you'll have it show 2.7K. But this temperature is not a property of space, but of the radiation, that the thermometer comes thermal to equilibrium with.

4. Nov 5, 2018

Graeme M

Ah, so I wasn't mistaken. Space doesn't have a "temperature" itself. Thanks for that.

Re the sense in which radiation has a temperature, it's more that any particular radiation has a spectrum that matches that which would be emitted by an object at some corresponding temperature, is that right?

5. Nov 5, 2018

Bandersnatch

Yes, but this only works for black-body radiators.

6. Nov 5, 2018

Orodruin

Staff Emeritus
A photon gas has a temperature. The CMB is a photon gas.

7. Nov 5, 2018

Graeme M

OK... Now that is beyond me, but thanks all the same. Given that temperature in my limited understanding is a measurement of some energy state of a system, I'd have thought the "information" used to derive a temperature from a radiating object would be transferred by photons. Is that the case? If it is then In effect when we measure a temperature we are really observing the state of those photons (or more exactly the state of our measuring device as affected by those photons) wouldn't we? So... does that mean any set of photons at the right wavelengths has a "temperature"?

8. Nov 5, 2018

Staff Emeritus
Where?

Without seeing what was written originally, we can't tell if it is right or wrong or weather you are misunderstanding it,.

9. Nov 6, 2018

Graeme M

Yes, I had misunderstood. I was thinking about the fact that the universe ("space") must be filled with radiation from a variety of sources and so objects would warm at all sorts of different rates. When I was googling it I kept seeing the statement that the background temperature is 2.73K. While this was clearly stated as due to the CMB, my confusion was more due to the idea that the CMB "has" a temperature. As I can see now, it's not being said that the CMB itself has a temperature, merely that it is sufficiently energetic to warm objects to 2.73K. As explained earlier in the thread above.

As an aside, if thermal radiation is a mechanism for transferring energy between objects, wouldn't the CMB - originally emitted at 3000K - eventually have been completely absorbed by colder objects? I mean, I assume that the Big Bang is not a constantly radiating event so it kinda seems to me that the CMB would have warmed things over time and have become progressively weaker until it ceased to exist?

10. Nov 6, 2018

jbriggs444

The CMB was emitted just as the contents of the universe became transparent to its wavelength. The emitted radiation has not been absorbed since. This took place at a particular time. However, the CMBR persists because it was emitted everywhere. As time passes, we see the CMBR that was emitted from farther and farther away.

11. Nov 6, 2018

Graeme M

I hope I don't sound too dense by asking this, but why has the CMB not been absorbed since? My grasp on this stuff is pretty thin, but it's not immediately obvious to me why this particular thermal radiation hasn't been absorbed by colder objects over time.

12. Nov 6, 2018

Bandersnatch

There's just not enough intervening material in space to attenuate most of the radiation. The vast majority of photons emitted at recombination have travelled unobstructed, without ever encountering anything in their path.
Most of the universe is empty.

13. Nov 6, 2018

Graeme M

Ah, I see. Thanks for that. How interesting, I've never really thought about this before and only really knew very dimly about the CMB. So does that mean that at all times in the universe, no object could have been cooler than the CMBs energy profile, therefore for example at some time (maybe billions of years ago) all objects had to be warmer than say 375K?

14. Nov 6, 2018

Bandersnatch

I think so, yes.

15. Nov 8, 2018

Graeme M

While I knew of the CMB I've never really read up on it so have only a basic idea about it and this discussion is very enlightening. The idea of a photon "gas" is strange to me but I assume the point is that rather than the radiation being detected directly following emission from a distant object, this radiation is simply everywhere without a direct causal source - in a sense, the entire universe is simmering. In terms of detection though, we are really just detecting a local phenomenon. How do we know this radiation is a large scale phenomenon and not just a feature of local space? Is it because it matches predictions of the standard big bang model or do we have observations that point to its presence in more distant locations?

16. Nov 8, 2018

jbriggs444

You seem to be thinking of a diffusive real gas like the air we breath where mean free paths are short and viscosity plays a role. A photon gas is more ideal. Collisionless and with zero viscosity. https://en.wikipedia.org/wiki/Photon_gas

The CMBR you see here is the CMBR that was emitted way over there, a long time ago. It is not a fuzzy cloud that was created locally and is waiting for you to stick an antenna up to sense it.

17. Nov 9, 2018

Graeme M

No, but it is a "fuzzy cloud" that exists locally waiting for us to stick up an antenna to detect it, isn't it? I mean, isn't that all we can ever do - detect local radiation (local photons). With say light from a distant star, we can infer a causal chain between the light we have detected here and the distant star and therefore assume that the local photons came from that distant source and make judgements about the source. But if the CMBR is the field of thermal radiation from the time the universe became transparent to that radiation, none of it has a particular source. It just is, and all we can detect is that radiation which currently is local to us. I wondered how we know that it is necessarily a feature of the whole universe rather than some local feature. I just did a bit of a dig and found that it is assumed that these photons were emitted from a "surface of last scattering" which I think just is the time at which the universe became transparent. Sooo... I guess that's all too hard for me to follow. Is there independent evidence for the CMBR's origin other than its match to predictions of the big bang? Or is that enough to be pretty much certain?

18. Nov 9, 2018

jbriggs444

We can detect patterns in it. That would not be possible if it were sourceless. http://www.astro.ucla.edu/~wright/CMB-DT.html

19. Nov 10, 2018

Graeme M

Thanks jbriggs... I think!

What does the article mean when it says the "...velocity of the observer with respect to the Universe" and "...the Solar System is moving at 368+/-2 km/sec relative to the observable Universe"?

20. Nov 10, 2018

Bandersnatch

Just as one can pick a convenient frame of reference to describe events on Earth (e.g. w/r to the motionless ground, or w/r to the car I'm riding in), one can also pick a convenient frame to describe events in the universe.
A natural choice in the second case is the frame in which the CMB radiation looks the same in every direction, i.e. stationary w/r to the average motion of the gas that emitted CMBR. That's the reference used in the article.