How Do We Know the Cosmic Microwave Background Radiation Isn't Local?

[anya2]

Current cosmology is based on CMBR and the idea it is remnant from the big bang, I guess we assume it's the same radiation in the whole universe.

My question is how do we know the CMBR is not something local to our galaxy let's say? How do we know in another galaxy the CMBR won't be different?

We don't have the possibility to take measurements outside the solar system, not to mention the galaxy. So how do we know the CMBR is uniform and identical throughout the whole universe?

It's a bit like being in the middle of the pacific ocean, it will be just water and the horizon everywhere you look. Of course that doesn't mean the whole planet is like that, only that region, that happens to be everything within our range.

How do we know it's not the same with the CMBR? Recently I've read that there are other possible sources of the CMBR like giant supernovae scattered radiation or super compressed matter that falls into the event horizon of black holes, that pretty much resembles conditions, similar to those in the beginning of the universe according to the bb theory.

 

[S.Vasojevic]

Hi anya2, welcome to PF.

CMB is same in every direction. You can point dish towards our galactic center, or away from it, you will register same amount of radiation. In fact to guys who first noticed it, it looked like some system error, because it is so uniform in every direction. Special instruments needed to be developed, just to be able to measure small differences.

Any other source, other than 'leftover' from Big Bang, simply could not provide such uniformity. Even if scattering is not a problem, there would not be enough time for CMB photons to fill space with such uniformity.

 

[anya2]

Well, if the source is the massive black hole in the center of the galaxy - the galaxy has only 60000 ly, so it has plenty of time to travel well past our point of observation. The field that would extend is so big it can easily seem like it's uniform, as our point of observation is like a grain of sand compared to an ocean. The analogy I posted before - even if we "see" or measure the same thing in every direction does not mean that's all there is, it simply suggest that our range and our sensitivity cannot detect anything more.

Also, isn't it possible for every black hole to have a specific signature of the radiation it produces? Like pulsars and quasars do? Isn't it possible for the CMBR to encapsulate the specific configuration of our galaxy instead of an early universe? Isn't it possible for different galaxies to have their own specific signatures, but too faint to accurately measure by our equipment? Or mistaken for anisotropies for that matter?

All I want to say is how can we be sure of what is outside our galaxy, we have never been there, we haven't taken measurements from an external point of observation. There are many examples of things that appear uniform and spread to infinity, that only appear so due to our observation skills.

Isn't assuming all those things about the CMRB the "easy way out" - a form of convenient view that solidifies established theories? I guess if it didn't fit that well it would be investigated much more than it is today. In my opinion it is not that good idea to assume such a great deal about something as fundamental to cosmology as the CMBR.

And then, there is the COLD SPOT - if indeed the CMBR is remnant of the big bang and a truly homogeneous and uniform universe - there shouldn't be such gaps. Scattering in an early universe should not produce such cold spots as well. It is impossible for such a gigantic void to occur that soon after the big bang to account for this anomaly. What would have caused such a void in a homogeneous universe?

 

[S.Vasojevic]

Well, if the source is the massive black hole in the center of the galaxy - the galaxy has only 60000 ly, so it has plenty of time to travel well past our point of observation. The field that would extend is so big it can easily seem like it's uniform, as our point of observation is like a grain of sand compared to an ocean.

Yes, but then we would be able to pinpoint the source with ease, like any other source of EM radiation.

And then, there is the COLD SPOT - if indeed the CMBR is remnant of the big bang and a truly homogeneous and uniform universe - there shouldn't be such gaps. Scattering in an early universe should not produce such cold spots as well. It is impossible for such a gigantic void to occur that soon after the big bang to account for this anomaly. What would have caused such a void in a homogeneous universe?

Cold spot is actually about 70 micro Kelvins colder than average, and it is just the region with highest deviation. It can still fit statistics with about 2% of probability for such region to exist.

 

[DrChinese]

Isn't assuming all those things about the CMRB the "easy way out" - a form of convenient view that solidifies established theories? I guess if it didn't fit that well it would be investigated much more than it is today. In my opinion it is not that good idea to assume such a great deal about something as fundamental to cosmology as the CMBR.

Now you are moving into the philosophy of the scientific method. In essence, you are saying that accepting the CMBR a) requires special proof; and b) is biased because it supports established theory. Both of these positions have problems.

a) Any supporting evidence is evidence, it is simply a matter of degree. There are 2 important points that mitigate: i) The hypothesis of the CMBR led to numerous predictions which were subsequently verified; and ii) the hypothesis had substantial additional independent corroboration in the form of Hubble's Law.

b) The Big Bang was not a fully accepted theory at the time the CMBR was discovered. It was a strong candidate that became a lot stronger.