Jump to content

The CMB, how is that possible?


Recommended Posts

It may be that an observer 10 billion years in the future would also observe the same things than we do, and conclude exactly the same, that the Big Bang occured 13 billion years from him.

 

And what evidence do you have for any of that speculation? None, I suspect.

 

As opposed to the big bang model which has mountains of evidence.

Link to post
Share on other sites

I think you need to re-think this elfmotat.

The observable universe has a horizon, and over time, far-flung potions of our universe pass outside this horizon.

But just inside this horizon, information still has a chance to reach us.

Next year, this point just inside the current horizon, will have past outside and become unobservable.

The time may come when everything outside our local group passes out of the horizon.

 

But every point still inside the horizon was once at approx. 4000 deg. and some of those photons are coming towards us.

Because the separation keeps increasing. the red shift increases. Even after the point of origin has passed outside the horizon, the photons are still on their way. Every year more and more of these photons lose the ability to reach us but there are many more which started off closer. That is why I used the exaggeration that, eventually after billions of billions of years, the only CMB radiation may have originated from your neighborhood.

 

The exit of the CMB from the observable universe could only happen if the CMB was of a spherical nature and surrounded ( a portion of ) the observable universe, i.e. a fixed distance from us. Only then could it expand out of the horizon. But that is not the case, the CMB is everywhere.

Link to post
Share on other sites

I say that any random observer should observe roughly the same thing,should establish mathematically the same laws of physics and should conclude the same things about the universe.

 

On what basis do you assert this? Identical laws do not imply identical results.

Link to post
Share on other sites

The exit of the CMB from the observable universe could only happen if the CMB was of a spherical nature and surrounded ( a portion of ) the observable universe, i.e. a fixed distance from us. Only then could it expand out of the horizon. But that is not the case, the CMB is everywhere.

 

The source of the CMB we see is a sphere with us at the centre. As the photons from that surface reach us, the surface moves away (because we have already seen the photons from the old position). That is why I think Lineweaver's analogy posted above is so good.

Link to post
Share on other sites

Your under a few misconceptions. We do see objects beyond the Hubble sphere those objects have an apparent recessive velocity of 3c at z=1090. The universe expands at the same rate per volume given by Hubbles constant. Roughly 70 km/s/Mpc this rate of expansion is far slower than the distance light can travel in one second. So locally light has no problem overcoming the rate of expansion. Recessive velocity itself is a distance dependant value Hubbles law states the greater the distance the greater the recessive velocity. v=Hd. There is also thermodynamic evidence that supports expansion the ideal gas laws apply to cosmology. The temperature of the CMB was 3000kelvin today the universe is 2.73 kelvin. This temperature drop is explained by the ideal gas lawsas the volume of the universe expands the temperature and density drops. The CMB is a result of particles dropping out of thermal equilibrium and being able to form stable reactions due to that temperature drop of far higher temperatures prior to the CMB. Here is a good paper covering the thermodynamics

Edited by Mordred
Link to post
Share on other sites

But every point still inside the horizon was once at approx. 4000 deg. and some of those photons are coming towards us.

 

The CMB photons that we see all come from well outside of our local group. The photons that were emitted anywhere close to us were either absorbed or flew by.

Link to post
Share on other sites

I haven't read it completely but I believe Mordred's second link explains why the CMB, no matter how faint it becomes, will never 'recede' outside the horizon.

Link to post
Share on other sites

Correct though not it's set intention it also applies to other objects beyond the Hubbles sphere

As side note I was one of the authors proof readers for his his dissertation his knowledge and skills are far beyond me now. His current research includes the CMB in regards to inflation

They can be found on arxiv.org

So if anyone has comments or recommendations on the article I can contact the author

Keep in mind there is a difference between Hubbles sphere and the cosmological event horizon. Hubbles sphere is the age of the universe * c

 

The older FLRW metrics thought light could not reach us beyond the Hubbles sphere. Some older textbooks on the market today still apply those related metrics. If the metrics does not include the cosmological constant it is out of date.

 

We can see objects beyond the Hubbles sphere for the reasons covered in that article which targets the common forum misconceptions regarding expansion. The author is a member on another forum which is where I met him. He wrote the article based on the common forum misconceptions

Edited by Mordred
Link to post
Share on other sites
  • 1 month later...

Here is the question

 

At the Big Bang, the universe were created and started expanding. The universe includes us, the Earth, the Milky way galaxy, and also the CMB.

So the question is: how is it possible for us to observe a radiation that was emitted at the time we were born?

It looks like the archer running faster and being killed by its own arrow.

 

Hi michel123456,

 

As you know all radiation is believed to be stretched out based upon expanding space, this accordingly being the reason EM radiation becomes longer and redshifted over time. The recombination era was supposedly a hot part of the beginning universe and the theoretical source of the MBR; its radiation accordingly has been vastly redshifted since then. Expanding space is considered synonymous with an expanding universe. So the microwave background is not moving with us, it is theoretically absorbed and re-radiated in all directions which we observe as a background temperature. Since according to the BB model the recombination era was the source of this radiation, it accordingly would be observable in all directions from us.

 

As you also know other cosmologies have proposed other explanations to explain the MBR.

 

http://en.wikipedia.org/wiki/Cosmic_microwave_background

 

Edited by pantheory
Link to post
Share on other sites

 

The universe was ~380,000 years old when the universe became transparent

 

What was the size of the universe at that time ?

I don't know, but it must be a lot smaller than today, and I can suppose that the CMB had time to pass by us.

May be the CMB we detect now is just the echo of the photon that was absorbed and reemited and reabsorb etc...

Link to post
Share on other sites

What was the size of the universe at that time ?

 

Good question. I don't know. Much, much smaller than now. But, because of expansion, it has taken those photons about 13.7 billion years to get here. The surface where those photons were emitted is now about 46 billion light years away.

 

May be the CMB we detect now is just the echo of the photon that was absorbed and reemited and reabsorb etc...

 

As I understand it, that would not give us the near-perfect black body spectrum we see.

Link to post
Share on other sites

 

What was the size of the universe at that time ?

I don't know, but it must be a lot smaller than today, and I can suppose that the CMB had time to pass by us.

May be the CMB we detect now is just the echo of the photon that was absorbed and reemited and reabsorb etc...

 

 

Good question. I don't know. Much, much smaller than now. But, because of expansion, it has taken those photons about 13.7 billion years to get here. The surface where those photons were emitted is now about 46 billion light years away.

 

 

As I understand it, that would not give us the near-perfect black body spectrum we see.

 

The temperature of the era of last scattering was around 3000 kelvin - the cmbr is now 2.7 kelvin. The drop in temperature is due to the red shift caused by expansion and is of the same ratio - this scale factor is about 1100. So what is 46 billion light years across now was about 44 million light years across then.

 

And re the black body spectrum - that is my understanding as well

Link to post
Share on other sites

 

 

The temperature of the era of last scattering was around 3000 kelvin - the cmbr is now 2.7 kelvin. The drop in temperature is due to the red shift caused by expansion and is of the same ratio - this scale factor is about 1100. So what is 46 billion light years across now was about 44 million light years across then.

 

And re the black body spectrum - that is my understanding as well

How far back would we need to go for the CMBR to be visible to the naked eye, both in terms of wavelength and intensity?

Link to post
Share on other sites

How far back would we need to go for the CMBR to be visible to the naked eye, both in terms of wavelength and intensity?

The black body spectrum at 3000k would be visible - the colour/tone it would be like soft fluorescent tubes. So that is at formation we could see it. And as it makes up the majority of the radiation energy of the universe and about a millionth of the total density of the universe then I hope the intensity would be high enough.
Link to post
Share on other sites

For a solid, the Draper point is 798K — that's where incandescence becomes visible.

http://en.wikipedia.org/wiki/Draper_point

 

If the intensity was sufficient, you would see a red tinge, though most of the light would be in the IR. So, however long it took to cool below 800K is when the CMB became invisible to us.

Link to post
Share on other sites

 

 

The temperature of the era of last scattering was around 3000 kelvin - the cmbr is now 2.7 kelvin. The drop in temperature is due to the red shift caused by expansion and is of the same ratio - this scale factor is about 1100. So what is 46 billion light years across now was about 44 million light years across then.

 

And re the black body spectrum - that is my understanding as well

So if the universe was 44 million light years across, we should have received that light a little bit more than 44 millions years latter even with the expansion of space. So the CMB is not photon directly received from that era but more echo.

Link to post
Share on other sites

So if the universe was 44 million light years across, we should have received that light a little bit more than 44 millions years latter even with the expansion of space.

 

No, it took 13.7 billion years to get here because of the the expansion of space. You can think of it (crudely and incorrectly) as it being dragged backwards. It is more accurate to say that when it started, it only had 44 million light years to travel but after it had travelled half that distance, the remaining distance had increased enormously. By the time it had travelled half the remaining distance, the distance had increased a bit more. And so on.

Link to post
Share on other sites

So if the universe was 44 million light years across, we should have received that light a little bit more than 44 millions years latter even with the expansion of space. So the CMB is not photon directly received from that era but more echo.

 

Nope - your assertion is incorrect. The universe expansion was very rapid - the gaps between things got bigger almost unimaginably quickly.

 

xposted with strange

Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.