Author Topic: Why can we still see the Cosmic Microwave Background?  (Read 17272 times)

Alice

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Why can we still see the Cosmic Microwave Background?
« on: April 04, 2009, 01:45:20 pm »
Answering somebody's question recently about where the Big Bang took place got me thinking.

To recap, the Big Bang took place everywhere in the Universe. It wasn't an explosion into the Universe, but the creation of it. I don't find the balloon blowing up analogy very useful because that still implies an edge. I like the idea of a Universe which doesn't have a centre or an edge - then we don't need to worry about what it's expanding into! Or am I making a mistake here?

Anyway, that wasn't the point.

In every direction we look, up or down, right or left, in front of or behind us - we see the Cosmic Microwave Background. It's 13.4 billion light years away. That's a long way. Of course, if we could get up close to it, we'd have gone back in time. We see the earliest galaxies and we're looking back in time. We look into history when we look into space, because light takes time to get somewhere.

So it's logical that the Cosmic Microwave Background is probably similarly everywhere, but also no longer in here-and-now existence. (Yes, I know there's no universal "now" in astronomy . . .) If someone in Andromeda, or any other galaxy "right now" (whenever that is) looked at the Cosmic Microwave Background, presumably it would be 13.4 billion light years away.

So it's in a different place in time rather than in space. Am I right?

In that case, how can we still see it? Yes, the light waves have travelled a long way, but if the Big Bang was here, how did they travel across such a long distance? Are we seeing a faraway bit of CMB - faraway as in, when our bit of space was newly born, it was a long way away? Was this due to inflation?

I am sure I have got something very wrong here. Can anyone enlighten me? I'd be grateful :)

NGC3314

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Re: Why can we still see the Cosmic Microwave Background?
« Reply #1 on: April 04, 2009, 01:56:33 pm »
What we (or anyone else) see as the CMB at a particular time is a spherical slice of the Universe, with radius such that the light has travelled 13.7 billion years (more precisely known, age of Universe minus 380,000 years). This slice or bubble expands at the speed of light a the Universe ages. Of course it represents material that was relatively close to us when the light left, but it's taken a lot longer for the radiation to get to us across the expanding Universe. So every location is in the middle of its own CMB sphere. Cosmologists wold love to sample someone else's, because there is a certain statistical error in properie sof the CMB whch is associated with only being able to sample one location in the Universe at one time (so-called cosmic scatter).

Alice

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Re: Why can we still see the Cosmic Microwave Background?
« Reply #2 on: April 04, 2009, 06:31:51 pm »
I love your pearls of wisdom, Bill. 8) 8) Thank you :)

. . . every location is in the middle of its own CMB sphere . . .

That is just magic! ;D

Why did the light take so long to get here, if it was close to us at the time? The expansion of the Universe?

sysboy

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Re: Why can we still see the Cosmic Microwave Background?
« Reply #3 on: April 04, 2009, 07:30:59 pm »
It wouldn't be magic if there was no edge to the universe. If you imagine the surface of the good old balloon for example.


Mark OConnell

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Re: Why can we still see the Cosmic Microwave Background?
« Reply #5 on: April 16, 2009, 09:06:35 pm »
If we exert an outside force to rapidly pull apart our universe do we not at the same time accelerate time in those regions while they are being expanded?

Edd

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Re: Why can we still see the Cosmic Microwave Background?
« Reply #6 on: April 17, 2009, 01:04:39 pm »
If we exert an outside force to rapidly pull apart our universe do we not at the same time accelerate time in those regions while they are being expanded?

Everywhere's the same (at least in the model we usually work in - whether that is right is another question), so there's no "in those regions". Even if you did have expansion in some parts that were different in others, I'm not sure it would result in "accelerated" time. Slower time though, perhaps. I'd have to think about it more before committing to an answer ;)

Also, I'm regularly picky about this - but light travel time is not the same as distance. You have to remember that the universe is a lot bigger now than it was, so that first light year the light covered is no longer only a light year big.

As a result, where that stuff emitting the CMB is now is a lot more than only 13-or-so billion light years away.
When I look up at the night sky and think about the billions of stars out there, I think to myself: I'm amazing. - Peter Serafinowicz

Mark OConnell

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Re: Why can we still see the Cosmic Microwave Background?
« Reply #7 on: April 17, 2009, 02:29:42 pm »
Here is why I ask that question. When space contracts as in what happens in a black hole time slows down(which I think allows for matter to stack infinately at a single point in space yet not touch because they are always 1 (for lack of a better word) degree different in space-time off from each other). If time and space are so intimately connected then if we have an expansion take place would not time pass faster within the expanse? Certainly it would not be precieved by those within the expansion. But if some places expanded faster then others you would have time pass faster within that space then in others.

Edd

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Re: Why can we still see the Cosmic Microwave Background?
« Reply #8 on: April 17, 2009, 02:37:02 pm »
Light leaving the vicinity of a black hole gets redshifted.

Light passing through an expanding region of space gets redshifted.

That and the measurement of the speed of running clocks from those regions are not unrelated effects.

If the measured time were going faster, you'd see a blueshift, not a redshift.
When I look up at the night sky and think about the billions of stars out there, I think to myself: I'm amazing. - Peter Serafinowicz

Mark OConnell

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Re: Why can we still see the Cosmic Microwave Background?
« Reply #9 on: April 17, 2009, 03:00:17 pm »
Maybe it's time to for me to watch those videos on relativity again.

Zeus2007

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Re: Why can we still see the Cosmic Microwave Background?
« Reply #10 on: April 18, 2009, 08:52:56 pm »
It so much easier and simple when we were in the stone age, we could just look at the stars and marvel at their beauty.  Now with all this light stuff is like adding an out of shape piece to a puzzle that's already complicated without it. :o
"The journey of a thousand miles must begin with a single step"
Lao Tzu, Ancient Chinese Philosopher.

fifelad55

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Re: Why can we still see the Cosmic Microwave Background?
« Reply #11 on: May 02, 2009, 03:47:10 pm »
The cosmic background radiation has, if my memory serves me right, has a temperature of 2.7 Kelvin. I therefore calculate that the wavelength of this radiation is now about 1.07 mm, which takes it into the microwave band of the electromagnetic spectrum.

I have a vague (very vague) understanding of the expansion of the universe. I read somewhere that the size of the observable universe is now estimated to be about 42 billion light years but I'm uncertain as to whether this is 42 billion light years in all directions or 42 billions light years in total.

Im planning on attempting a BSc in astronomy by distance learning with the University of Central Lancashire from October so this question should become clearer in time. There are no exams, which is a great help to me as it is now some 28 years since I last sat an exam in earnest and I doubt whether I still have the ability to sit through a number of 3 hours exams! Also, I live in Thailand, in an area where the light pollution is so bad that the limiting magnitude is only around 2.5 on a good night!

Alan


EigenState

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Re: Why can we still see the Cosmic Microwave Background?
« Reply #12 on: May 02, 2009, 11:15:24 pm »
Greetings,

The cosmic background radiation has, if my memory serves me right, has a temperature of 2.7 Kelvin. I therefore calculate that the wavelength of this radiation is now about 1.07 mm, which takes it into the microwave band of the electromagnetic spectrum.

I find this reasoning rather backwards.  The observations are that the CMB has a spectral distribution that is effectively identical to that of a classical black body radiator having a temperature of approximately 2.7K.  Naturally, the frequency of that radiation peaks in the microwave.  Thus, utilizing Planck's law, the spectral distribution leads to some "temperature"--not the other way around.

But to argue that the CMB is characterized by a conventional temperature--one predicated upon average kinetic energy--is just not valid.  The temperature citation is only meaningful viv a vis that of a black body radiator.

Best regards,
EigenState

NGC3314

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Re: Why can we still see the Cosmic Microwave Background?
« Reply #13 on: May 03, 2009, 09:37:02 pm »
The observations are that the CMB has a spectral distribution that is effectively identical to that of a classical black body radiator having a temperature of approximately 2.7K.  Naturally, the frequency of that radiation peaks in the microwave.  Thus, utilizing Planck's law, the spectral distribution leads to some "temperature"--not the other way around.

But to argue that the CMB is characterized by a conventional temperature--one predicated upon average kinetic energy--is just not valid.  The temperature citation is only meaningful viv a vis that of a black body radiator.

Best regards,
EigenState

I feel like jumping in here to say the the temperature of a blackbody radiation field isn't arbitrary and remains closely related to the kinetic (ordinary) temperature not only of the emitting material, but to another blackbody object (OK, that's an idealization) which comes to thermal equilibrium with this radiation bath. The temperature changes but stays well defined as the blackbody radiation redshifts, because the blackbody curve has the interesting property that not only does the spectral shape redshift to a new temperature, but the absolute normalization does so as well. So a blackbody allowed to come to equilibrium with the CMBR today would indeed come to a kinetic temperature of 2.7 K.

graham d

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Re: Why can we still see the Cosmic Microwave Background?
« Reply #14 on: May 04, 2009, 12:59:13 am »
Hello Fifelad-Ned Wright's tutorial will help
http://www.astro.ucla.edu/~wright/cosmolog.htm

There are two approaches re- energy carriers and their chronologies. The CMB epoch is quite late and in terms of energy densities the universe is passing from a radiation dominant era to the subsequent matter dominated era even though the total energy may be zero. We can all agree that if the system is at perfect equilibrium then forget any expectation that one can derive any prior history as to the evolution of that system.

There is no such thing as pure energy in isolation. Two angled high energy laser beams shine through each other and they don't interact. Very occasionally, if the enrgy of the photons is enough, high frequency gamma, then an electron and a positron are created out of pure energy. Deflected in a magnetic field they leave spiral tracks.That's a fact, proved experimentally.
 
Accelerated electrons and positrons accelerated to 2 TeV  create quark jets and ca.100 GeV quark antiquark pairs, all three generations although the electrons don't carry the colour charge.

So the world is made up of particles force fields and the carriers of the force eg. the photon of the electroweak that incorporates the electromagnetic field and gluons, both rest massless, and the electroweak W+- and Z0 very massive carriers.

Photons have the h*nu energy, particles carry E=mc2 rest mass energy. The latter ie. all particles that have mass charge carry momentum and  kinetic energy.
Well before the CMB event there's the accepted theory of near perfect thermal equilibrium between particles and photons, ca 1 billion to 10 billion photons for each particle? Which particle? Even in the sun's core it takes on average 5 billion years before, via the electroweak theory before a proton flips to a neutron via the W+- interaction that involves an electron and a neutrino. The latter particle will travel through a light year of matter as dense as lead before a 50/50 probability of interaction again! At the earliest of times, still quite late on by a hundredth of the first second; there are particles largely electrons/positrons and neutrino/antineutrinos in near thermodynamic equilibrium. The kinetic energy of even these neutrinos were busily engaged in particle pair production. A real black body container has a wall . Conceptually, we can create a real or ordinary matter container with a perfect vacuum inside, ie. no particles. OK- there are vacuum fluctuations but any particle is always produced in pairs, matter and antimatter particles in pair, hence they are called virtual particles. If the energy density is great enough real particles are created from the photon emissions from the wall and the energy density depends on the T to the 4th power and distribution onthe socalled Wien's displacement law. But to make a real particle that we observe, that is stable, requires the parity violation reaction of those massive carrier particles, the W+- and the Z0 if we are interested on where the inertial property of one of the Higgs particles. These exist for ca. 10-23second. So set against this timescale those departures from thermodynamic equilibrium 380,000year later. The creation of a particle pair at 2TeV or the new 14TeV Cern"condenses" a lot of mass energy in a small space. The departure from perfect equilibrium in an expanding space, but not so for the inner dimensions of particles needs to be extrapolated another 6 orders of magniyude for kinetic energies where the Forces of the electromagnetic field and electroweak field merge, then perhaps another 4 magnitudes higher for the strong force interactions. These energies are  ten orders of magnitude beyond Cern's capabilities but still the relation holds; not pure energy but those photons and gluons exchanging energy with particles whose kinetic energies  are related to that Planck relation and the distribution for lambda, ie. where the radiation peaks in relation to a temperature you can't really comprehend in the approach to the Planck Temperature. Particles and "pure" energy go hand in hand at all times and the temperature or what kind of temperature provides information about when those particles appeared. I hate to mention it but the appearance of the most abundant form of matter has a role to in its interaction with those massive  bosons. Ordinary matter irrespective of form utilises the same massless carriers the photon and gluon despite the disparity of mass charge whether its electron, neutrino or quark mass charge. The neutral hybrid or Z0 is an admix that through the conceived Higgs doublet potentials confers mass not just to ordinary matter particles but also to DM, whatever that is. Given a different electric charge the W electric charged bosons would still interact with the same photon and gluon carriers to generate a new particle set whose abundance  might be enhanced relative to normal matter particles at their defined appearance threshold temperatures. A lower electric charge means a lower internal energy and a greater abundance ratio for DM. By the time of the CMB event for ordinary matter, the chicken egg dilemma is possibly semantics. It's the kinetic energies of particles against photon energies that cannot be reversed since neutrino energies decoupled well before that event. Those 1 in a few thousandths departures from equilibrium, tiny as they are and subject to some error? are why those vacuum fluctuations in the first 10-23second mean there is still a story to be discovered. Perfect equilibrium would mean any discussion earlier than ca. 380,000 years post BB would be futile to contemplate, apart from an experimental  neutrino background temperature corroboration with the 40% lower temperature viz. CMB temperature from theory. It's remarkable that the predicted Higgs mass comes in at similar mass to the W and Z0 carriers, not as a consequence to the discrepancy in the weak mixing angle , now that the more hugely top quark was determined, but that the discrepancy is to wholly account for the Z0 mass . In nature, where one measures the ratio between particle pairs of different parity, it could be a left to right hand form of a molecule, those ratios are numerically similar, by that I mean an equilibrium ratio of 1 to 1 or 10/1 etc not many orders of magnitude different. The alledged DM/OM ratio is ca. 5 to 1. That cries out to me that the temperatures are similar and that inner symmetry energy differences are minor ie. several eV's not millions eV. Why are the W , and Z0 so similar and by inference the mass of the Higgs? It isn't on! There's another route to the neutral interaction whose minimal interaction vertex  gives rise to the DM Z0, as well as the inferred Higgs. It's the same Z0 perhaps. That's not to say that the Higgs isn't there.

Mass charge particle differentials are huge ; isospin and hypercharge are essentially constant. What else can change, the gravitational carrier; perhaps not. There's only onething left that can change if one assumes that the ratio of the alpha and g interaction are constant for the mixing angle formula ie. conserved. That's the electric charge. Were there two routes to Z0 then the equibrium ratio would be ca. 7.4 for the universal DM/OM ratio, ca. half decaying over a timescale of 13.7 billion years. So its not just as NGC Bill mentions as another but crucially anyother process one can rather might conceive. Regardless of how one defines the temperature of the process, the temperature doesn't matter, they are basically identical. Why wasn't the DM/OM ratio vastly different, look at the huge isotope variations; with DM it's a small number ratio that is saying - look here we have a partner in common. There are three infact, but a common Z0 in relation to identical and the same photon and g massless carriers. What might be the mass difference between a Wdelta- and a W-? minimal! But for an electrondelta- the mass difference is large. It would mean there's a weakly charged less massive electron out there, in addition to a full suite of all alloparticles differing only in electric charge; with common identical photon, gluons and the neutral weak force carrier Z0, two allo W's and the same alloquark suite, and same neutrinos. This is smaller than the supersymm doubling for normal matter by the way.

Apologies for DM p p pschychosis.

Basically, chronology comes first. Prior to CMB the particles and their momenta and kinetic energies with huge photon and neutrino excesses are there. Think of the particles as the walls. It's unidirectional towards the future although the Higgs needs a backwards in time component that can confuse things but support Eigen's assertion since kinetic energy is always positive either forwards or backwards in time.


« Last Edit: May 04, 2009, 01:17:04 am by graham d »