PDA

View Full Version : Time and the Microwave Background Radiation

sdsperth
2010-Jun-24, 10:32 AM
Is it theoretically possible to determine absolute time by measuring the wavelength of the microwave background radiation?

Cougar
2010-Jun-24, 01:15 PM
Is it theoretically possible to determine absolute time by measuring the wavelength of the microwave background radiation?

Welcome to the Board, sdsperth.

The CMB (http://en.wikipedia.org/wiki/Cosmic_microwave_background_radiation) has a blackbody spectrum (http://en.wikipedia.org/wiki/Black_body), so it's made up of lots of different wavelengths, peaking at 1.9 mm. And what do you mean by "absolute time"?

Ken G
2010-Jun-24, 03:38 PM
It sounds like you are asking if the concept of "age" of the universe can be used as a way to access "absolute time," and can you get at the age by looking at the CMB. First of all, you cannot get the age just from the CMB, you need to combine it with other observations too (like the Hubble law of expansion). Once you understand the expansion, the CMB just gives you the temperature of the universe looking backward, leading ultimately to Big Bang nucleosynthesis and other aspects of the self-consistency of the whole Big Bang model.

But there's also something deeper to your question, I believe. You are asking if, given that we can infer the age of the universe when various events occured, does that not give us a sense of absolute time? (Also, does it not give us a sense of a preferred reference frame, the frame in which the CMB shows no Doppler shifts?)

The answer there is, "it depends on what you mean by absolute." There's kind of a weak meaning, and a strong meaning, to that word. A weak meaning is just "a version of time that is picked out for special attention, a kind of convenience of thinking about time." A strong meaning is "built into the laws of physics themselves, in a way that is not just an accident of the initial conditions, but fundamental to the nature of reality." I would say that cosmic age is an example of the weak form of an "absolute" time, but not the strong form. To see this, you need to understand that it is only because of the "cosmological principle" (the idea that the universe is the same everywhere, but ages as a whole) that we can use a concept of cosmic age. This in turn may be a consequence of inflation early in the life of the universe, so it is not viewed as a fundamental law of physics. We can imagine universes that obey all the equations of physics, but don't exhibit the cosmological principle (which is not viewed as a fundamental law of physics).

I would say a better way to think about "real time" is the concept of proper time, which is time that is owned by an observer or clock (even if hypothetical). The cosmological principle gives us a set of "preferred" observers (think, more convenient or more generic observers, not observers singled out by the laws themselves), who are called the "comoving observers." These observers move with the average motion of the matter around them (on very large scales), and are the observers who see no Doppler shifts in the CMB (so we aren't comoving observers ourselves, but we're close, we're approximately so). The proper time of the comoving observers is what we call cosmic age. So it's still an arbitrary kind of time (since there are other observers who are not comoving, especially hypothetical ones), but it is preferred by the cosmological principle-- it is a simple coordinate to use when conceptualizing time in any universe that has a cosmological principle. Is an especially simple coordinate the same thing as an absolute quantity? Not in the normal meaning of "absolute."

AriAstronomer
2010-Jun-25, 01:53 PM
It is important not to become confused though sdsperth that there really is no 'absolute' time frame. Everything is always moving relative to something else, and therefore any event can always be viewed in a different manner depending on where you are. If you take Ken G's approach to the CMB though and define what you probably mean as 'an observer moving about the same as the objects around it' (on a large scale to negate doppler shifts), then we have a pretty good idea of exactly how old the universe is. To really speak of a 'absolute time', you would have to think of some God figure outside of the universe counting the seconds, or something that is moving w.r.t. to everything at the same time (which doesn't really make sense, at least in our 3-D world).

forrest noble
2010-Jun-26, 12:35 AM
sdsperth,

Time and the Microwave Background Radiation

Is it theoretically possible to determine absolute time by measuring the wavelength of the microwave background radiation?

Cool your very first posting and question on BAUT.

To start with you might have to give your own definition of absolute time to be able to answer your question. I'm not aware of a consensus of opinion concerning a definition for "absolute time. Today a second of time is defined by a cesium clock which has become an international standard concerning a unit of time. In the beginning there probably weren't any of these clocks around so you may choose a looser definition such as the sequence of changing events and milestones in the history of the universe. Then you could equate milestone events and periods with their estimated elapsed time periods in millions or billions of years. In seconds, or fractions thereof, you might wish to speculate concerning an original Big Bang scenario.

You might define absolute time one way, but you will probably find that there would be no consensus concerning your definition.

After you give your definition of absolute time next you need to state your objective. When you begin an analyses you need to make some assumptions. Maybe the primary one is that you must assume that CMB waves are truly embryonic as to their Big Bang source which is not a big assumption since it is assumed to be true by the Big Bang model. You may decide that you would have to see a difference in the micro-wave background between differing time periods according to their related redshift. Some have claimed that they have observed such things. If you believe that observations in the past supposedly have a different temperature of the CMB would you have met your objective concerning your understanding of absolute time? or is your objective broader than that?

sdsperth
2010-Jul-01, 12:01 PM
Thankyou everyone. Your answers have all been extremely informative and encouraging.

My question about "absolute time" came from reading in cosmology books that it is not possible for two observers to agree on what time an event occured, as there is no universal time keeper.

I wasn't thinking about how old the universe is, but rather if in theory and taking into account doppler shifts, whether I could observe and event, record it and timestamp it with the CMB wavelength (or as cougar points out, the peak wavelength).

Many thanks again.

Ken G
2010-Jul-01, 04:49 PM
I wasn't thinking about how old the universe is, but rather if in theory and taking into account doppler shifts, whether I could observe and event, record it and timestamp it with the CMB wavelength (or as cougar points out, the peak wavelength).You could indeed do that, and indeed that is pretty much exactly what is done. But the main point to get is, what you are doing is selecting a time coordinate, rather than using some absolute version of time. A coordinate is often chosen for convenience, and the cosmological principle makes this a very convenient time coordinate indeed, but a coordinate of convenience is a convention, rather than being a statement of absolute reality, which is what "absolute time" sounds like it is trying to be.

Spaceman Spiff
2010-Jul-02, 02:48 AM
Welcome to the Board, sdsperth.

The CMB (http://en.wikipedia.org/wiki/Cosmic_microwave_background_radiation) has a blackbody spectrum (http://en.wikipedia.org/wiki/Black_body), so it's made up of lots of different wavelengths, peaking at 1.9 mm. And what do you mean by "absolute time"?

Cougar -- a slightly off-OP comment, but still related to the CMB....
The position (wavelength, frequency, energy, wave number) of the peak intensity of a blackbody depends on the function that is being plotted. e.g.,

Watts/m2/nm vs. nm (wavelength)
Watts/m2/nm vs. cm-1 (wave number)
Watts/m2/Hz vs. Hz (frequency)
Watts/m2/Hz vs. nm (wavelength)
Watts/m2/Hz vs. cm-1 (wave number)
etc.

Of course, the above are in energy flux units, and one could also plot in photon flux units! :razz:

Quite possibly you are aware of the above and were just providing a quick answer, but in any case I point out the above for others.

Robert Tulip
2010-Jul-02, 03:04 AM
what you are doing is selecting a time coordinate, rather than using some absolute version of time. A coordinate is often chosen for convenience, and the cosmological principle makes this a very convenient time coordinate indeed, but a coordinate of convenience is a convention, rather than being a statement of absolute reality, which is what "absolute time" sounds like it is trying to be.

Related discussion is in this Science and Technology thread (http://www.bautforum.com/showthread.php/104931-What-is-the-meaning-of-the-claim-quot-time-is-an-illusion-quot?p=1754276#post1754276) on Is Time an Illusion.

Does the consistency of the cosmological principle require absolute time?