Background Radiation & Neutrino Questions

[Nim]

When the cosmic background radiation first appeared 300,000 years after the Big Bang, was it x-rays and then redshifted over time to microwaves?

And to see what the Universe looked like before 300,000 years after its birth, will we need a neutrino telescope?

 

[Labguy]

Originally posted by Nim
When the cosmic background radiation first appeared 300,000 years after the Big Bang, was it x-rays and then redshifted over time to microwaves?

And to see what the Universe looked like before 300,000 years after its birth, will we need a neutrino telescope?

No, we see it now as ~2.73 degree microwave radiation.

 

[jcsd]

Originally posted by Nim
When the cosmic background radiation first appeared 300,000 years after the Big Bang, was it x-rays and then redshifted over time to microwaves?

And to see what the Universe looked like before 300,000 years after its birth, will we need a neutrino telescope?

No, it was orginally in the lower frequency range of visible light.

A gravitational wave telescope would be better, but yes neutrino decoupling happened after about 1 second after the big bang and lasted for about 10 seconds compared to photon decoupling which started about 300,000 years after the big bang, so with neutrinos you should be able to see much further back than with the CMBR.

 

[marcus]

Originally posted by jcsd
No, it was orginally in the lower frequency range of visible light.

A gravitational wave telescope would be better, but yes neutrino decoupling happened after about 1 second after the big bang and lasted for about 10 seconds compared to photon decoupling which started about 300,000 years after the big bang, so with neutrinos you should be able to see much further back than with the CMBR.

A temperature of 1.9 kelvin has been predicted for the cosmic neutrino background CNB

which is currently not observable----those neutrinos are much lower energy than the ones we can detect with today's instruments

and the CNB is, according to Lineweaver's survey of cosmology which has a lot of useful information, in fact just as jcsd says, supposed to date from the first few seconds of expansion.


Lineweaver's article, which has the temperature estimate of 1.9 kelvin, is online at Caltech level 5 and also at

http://arxiv.org/astro-ph/0305179

 

[Tyger]

If the Neutrino temp is 1.9 degree and Neutrinos are massive then they are basically at rest. If they were tachyonic they would have circled around the universe a few times by now, assuming that it is closed.

 

[Nim]

We see it now as 2.73 degrees kelvin? So the temperature is going down?

 

[marcus]

Originally posted by Nim
We see it now as 2.73 degrees kelvin? So the temperature is going down?

Nim you really should get Lineweavers article. Download it.
It has a lot of good stuff

The MICROWAVE background is 2.73 kelvin and dates from 300,000 years after time zero

The neutrino background is older---going back to the first second of expansion. It is a good question why the temperature is predicted to be 1.9 kelvin. Lineweaver explains this in section 7.4 of that article

these are two separate cosmic backgrounds only one of which has been detected. It will be very exciting and informative when the cosmic neutrino background is finally detected and can be studied

I will edit in some Lineweaver links

http://nedwww.ipac.caltech.edu/level5/March03/Lineweaver/Lineweaver_contents.html

this gives you the TOC, the page you want is section 7.4 "Where did the energy in the CMB come from?"

http://nedwww.ipac.caltech.edu/level5/March03/Lineweaver/Lineweaver7_4.html

This is the same as what you get if you download it from the LosAlamos archives and print it out and look at page 24.

I mention the CalTech version because you can get at it without
having to download the whole article. But it is more legible if you
download it from

http://arxiv.org/astro-ph/0305179

 

[marcus]

Originally posted by Tyger
If the Neutrino temp is 1.9 degree and Neutrinos are massive then they are basically at rest. If they were tachyonic they would have circled around the universe a few times by now, assuming that it is closed.

Tyger, boltzmann's k (you are implicitly invoking this) is
8.6E-5 eevee per kelvin

So at 1.9 kelvin, the kT energy is 1.6E-4 eevee

If a neutrino has mass of 1 eevee, then how fast must it go
to have kinetic energy of 2E-4 eevee?

I think it must be going about 1/50 of the speed of light

What are some current upper bounds on neutrino mass? Or
estimates, if they have them?

 

[Nim]

I don't understand this temperature thing. When it comes to different frequencies of light, I thought that lower frequencies had a lower temperature and that higher frequencies had a higher temperature... I didn't think a frequency of light could loose heat.

 

[Hurkyl]

The light doesn't remain the same frequency; it redshifts, curtosey of the expansion of space.

 

[marcus]

Originally posted by Nim
I don't understand this temperature thing. When it comes to different frequencies of light, I thought that lower frequencies had a lower temperature and that higher frequencies had a higher temperature... I didn't think a frequency of light could loose heat.

keep asking Nim
do you know the black body curve---the spectrum
of the glow from a generic surface at a specific temperature?
each temp is associated with a distinctive blend of
frequencies of light

if something has a thermal spectrum----is glow characteristic
of some temp
and then you dopplershift or redshift every photon in the blend
what results is a thermal blend of frequencies but for a different
temperature------just as if the radiating thing were some percentage cooler

think about glowing plasma at 3000 kelvin
releasing light
and then space expands by a factor of 1000
so all the light is redshifted (wavelengths stretched out)
by a factor of 1000

the light still looks like thermal glow, but off a colder object.
It looks like it came from something that is only 3 kelvin
in fact it is microwave background or infrared or whatever you call it

keep asking
this makes it more interesting for Hurkyl and me and suchlikes

 

[Nim]

So the cosmic background radiation started out at about the red, orange, yellow part of the spectrum and redshifted down to microwaves from there? And some day it will be redshifted down to radio waves? How low of a temperature will the cosmic background radiation actually end up getting to anyways?

 

[Labguy]

Originally posted by marcus
What are some current upper bounds on neutrino mass? Or
estimates, if they have them?

Neutrino mass:

http://www.jupiterscientific.org/sciinfo/numasses.html

 

[jcsd]

Originally posted by Labguy
Neutrino mass:

http://www.jupiterscientific.org/sciinfo/numasses.html

If you looked at at a convential table of elementary particles you'd see that the (electron, as there are actually three known types of neutrino) neutrino's mass is given as zero. There have been a couple of experiments that seem to suggest that neutinos may have mass but partly due to the fact that scientists have been unable to replicate the results the coin is still up in the air.

 

[Phobos]

Originally posted by marcus
The MICROWAVE background is 2.73 kelvin and dates from 300,000 years after time zero

Just to clarify, the current "temperature of the universe" is 2.73K...the temperature back when the universe was only 300,000 years old was MUCH higher. (as the universe expands, it cools).

The 300,000-year milestone is when the universe cooled enough to became transparent to light.

 

[marcus]

Originally posted by Nim
So the cosmic background radiation started out at about the red, orange, yellow part of the spectrum and redshifted down to microwaves from there? And some day it will be redshifted down to radio waves? How low of a temperature will the cosmic background radiation actually end up getting to anyways?

expectations about the future depend on a choice of model
the simplest model that fits observational data has a positive
cosmological constant which, if it really is constant, implies
unlimited expansion

there is no lower bound on CMB temperature
it can get as close to zero as you please
as long as you are willing to wait long enough

there is enough information in Lineweaver's Figure 1
(a diagram showing assumed future of universe as well as past)
to let someone estimate how long we have to wait for
CMB to get down to 1.36 kelvin or half what it is today.

http://nedwww.ipac.caltech.edu/level5/March03/Lineweaver/Figures/figure1.jpg

Would you like to know how to read that from Figure 1?
If so ask.

If you have the article printed out (a lot more legible) then
you will find Figure 1 on page 6. Here is a link to the PDF
version you can print out.


http://arxiv.org/astro-ph/0305179

I think it is the best general audience
survey of contemporary cosmology I have seen so far.

 

[Nim]

Originally posted by marcus Would you like to know how to read that from Figure 1?
If so ask.

Yeah that would be great.

So if the CMB continues to redshift, at some point in time it will have a temperature of something like 0.000000000001 eventually and maybe a wavelegnth of a few miles. If the Universe never stopped expanding would the CMB finally just disappear one day? There is probably a limit to how low a temperature can be without being absolute zero right, or a limit to just how long a wavelength could be. I would imagine a wavelength of a trillion light years or a temperature of say 1e-999 would be impossible.

 

[marcus]

Originally posted by Nim
Yeah that would be great.

OK, figure 1 consists of three different plots of the past and future of the universe

look at the top one, which has ordinary years (Gyr = billion years)
on the left side

and notice that it has the "scalefactor a(t)" on the right side

the scalefactor is an index of the size of the universe which for convenience is normalized so that its value at the present is one
a("now") = 1

At some time in the future the average distance to galaxies will
be twice what it is now, things will be twice as spread out, the CMB wavelengths will be twice as long, its temperature will be half what it is now.

a("then") = 2

You can look across Lineweavers graph from the size scale on the right over to the time (Gyr) scale on the left and see when that will be measured in billions of years from the big bang

 

[marcus]

Originally posted by Nim
There is probably a limit to how low a temperature can be without being absolute zero right, or a limit to just how long a wavelength could be. I would imagine a wavelength of a trillion light years or a temperature of say 1e-999 would be impossible.

If the model is correct in predicting endless expansion then yes evenutally things will have expanded 1000-fold and instead of being measured in millimeters the CMB spectrum (power in each wavelength interval) will be measured in meters

the signal will be (1000)⁴ fainter due to expansion

eventually there is the philosophical issue---does something exist if it cannot be observed. this does not intrigue me personally but it may interest others

what do you suppose they mean by the "temperature" of some radiation?

I think that even if the wavelength of each CMB photon were stretched out 1000-fold that the CMB would still have a temperature as astronomers understand the word

but it might be very hard to detect the CMB and measure its temp

does it strike you as odd that I'm claiming the CMB signal would be (1000)⁴ times fainter (that is, a trillion times fainter)
after only a thousand-fold expansion?

 

[Nim]

Yeah, how exactly does a 1,000 fold expansion of the CMB make it a trillion times fainter? When you say the temperature of some radiation, are you talking about the incredibly small differences of temperature in the CMB?

If a wavelength can get endlessly longer or if a temperature can get endlessly closer to absolute zero, wouldn't that break some rule of quantum mechanics? Isn't there supposed to be a smallest piece of everything, a quantum unit of an electric charge, time, length, temperature, etc.

 

[marcus]

Originally posted by Nim
Yeah, how exactly does a 1,000 fold expansion of the CMB make it a trillion times fainter? When you say the temperature of some radiation, are you talking about the incredibly small differences of temperature in the CMB?

this only seems unintuitive, it is a routine fact in cosmology:
think of a cubic kilometer full of CMB photons
there is some number of them, say N

now expand the picture by a factor of 10 (easier to write than a thousand so let's just do it for ten)

the new volume is 10³ bigger and there are still only N photons in it so the density of photons is only a thousandth of what it was------plus each photon wavepacket is stretched out by a factor of 10 so has only one tenth the energy

the combined effect is the CMB energy density is only a tenthousandth.

the CMB energy density declines as the FOURTH POWER of the factor by which space expands

----------------
I suspect what you say about there being a 'smallest piece'
of something doesn't work for everything
or wait, let me be more circumspect in how I say it,
I haven't yet heard any evidence that there is a smallest
amount for every type of quantity. It is already surprising
to many people that in LQG the area and volume operators
are quantized. This from a rather new theory (Loop Quantum Gravity) not yet firmly established. the basic inputs interestingly enough are continuous and the discreteness comes out of the analysis----it is not one of those theories where one assumes
a latticework space to begin with. So the quantization of area etc is not "put in by hand" or on purpose-----it does not HAVE to come out of the theory but it does anyway. This is suggestive that area etc are really quantized at ultramicroscopic scale in nature. I doubt anybody really knows what to make of that.
But I would be reluctant to guess that
temperature is quantized merely on that evidence.