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drhex
2006-Aug-23, 09:00 PM
I'm thinking about the "Energy budget" of the universe. We've heard that it is roughly
5% ordinary matter
25% dark matter
70% dark energy

So where are the photons? They sure carry energy as one can feel on a hot summer's day. Are they perhaps included in "ordinary matter"?

neilzero
2006-Aug-23, 09:25 PM
Since ordinary matter and black holes sometimes absorb photons, perhaps 1%of the photons are included in ordinary matter. Most photons (a google of them?)however are traveling at the speed of light enroute to somewhere. Some have been enroute for 13.7 billion years. Neil

drhex
2006-Aug-24, 07:09 AM
Uhu? We can't claim to have a complete energy budget if 99% of the photons are not accounted for ?!

Ken G
2006-Aug-24, 10:44 AM
The photons are accounted for in the normal matter, yes, but it is a very small component (much less than 1%). There was a time, very early in the universe, when photons dominated the energy budget, but expansion is hard on photon energy. You see, rest mass energy density only gets diluted in inverse proportion to the volume increase, but photon energy has that effect along with the redshift effect, so its energy has gotten severely depleted with time. This is similar to what happens to the kinetic energy in a gas as you expand it adiabatically in a piston.

astromark
2006-Aug-24, 11:20 AM
Where are the photons ?

Look up on any clear night. There, that point of light that you call a star has just activated the cones inside your eye by the very photons you ask of.

No that heat you feel in daylight is not the energy of photons. It is the radiated energy of that fusion reactor we call the sun.

This universe has a great deal of photon producing stars. Sending out into the full 360 deg. some of this reaches planet Earth and you.

Is this the question or the answer? why am I confused.?

selden
2006-Aug-24, 11:34 AM
astromark,

I think another way to phrase what drhex was asking is "How much of the mass of the universe is in the photons that are still in transit? Why isn't that mentioned when people discuss how mass is distributed?"

ToSeek
2006-Aug-24, 02:27 PM
No that heat you feel in daylight is not the energy of photons. It is the radiated energy of that fusion reactor we call the sun.

But radiated energy = photons. What else would it be?

drhex
2006-Aug-24, 04:41 PM
Ok, Ken G, so the universe's original supply of photons has been diluted and redshifted down to insignificance and the new photons continuously produced by stars are also insignificant when compared to the entire energy budget?

pghnative
2006-Aug-24, 05:07 PM
Well, when hydrogen fuses to helium, less than 1% of its mass is converted to energy, at least by my quick and dirty math. Presumably the same ratio is true for the other common fusions.

This would mean that the energy in the form of mass is at least 99X the energy in the form of photons.

Ken G
2006-Aug-24, 09:30 PM
Ok, Ken G, so the universe's original supply of photons has been diluted and redshifted down to insignificance and the new photons continuously produced by stars are also insignificant when compared to the entire energy budget?

Bingo.

Ken G
2006-Aug-24, 09:34 PM
Well, when hydrogen fuses to helium, less than 1% of its mass is converted to energy, at least by my quick and dirty math. Presumably the same ratio is true for the other common fusions.

This would mean that the energy in the form of mass is at least 99X the energy in the form of photons.
This is also true, indeed even this 1% effect would hold only once all the hydrogen was fused to helium, a fate which so far has befallen only a tiny fraction of the hydrogen (most of it has never been in the core of a star).

CuddlySkyGazer
2006-Aug-25, 02:46 AM
To answer drhex's question: photons are indeed included in the ordinary matter of the Universe. 'Matter' does not just include things that have mass.

grant hutchison
2006-Sep-02, 04:35 PM
I happened on a figure for the photons' contribution to the composition of the Universe today, so thought I'd resuscitate this thread with the figure, gleaned from an article concerning dark matter written by David B Cline in a Scientific American special issue called Majestic Universe.
The ~1087 photons estimated to exist in the observable Universe at any given moment during the current epoch have an average energy per particle of just 10-4eV. This is to be contrasted with the estimated ~1078 particles of baryonic matter, with an average energy of 108eV. The total complement of baryonic matter therefore contains 1000 times the mass-energy of the photons. Matter contributes 5% to the total mass-energy of the Universe, and the photons 0.005%. Whether you leave photons out of the list altogether, or mix them in with "matter", their contribution is negligible for accounting purposes.

Grant Hutchison

Ken G
2006-Sep-02, 10:46 PM
Thanks Grant, those are useful numbers. I think protons are closer to an energy of 1 GeV though, so the numbers might be off a teensy bit, although you are probably counting the electrons and so getting 5 times 10^8 eV and using just 10^8, such are the vagaries of OOM estimates. My real question is, it might seem that there are a lot more neutrinos, so I'm guessing you are not counting those in the baryonic particle counts? A reasonable omission, since their mass-energy is expected to be fairly minimal.

grant hutchison
2006-Sep-02, 11:14 PM
They're all derived from figures in Cline's article. My only bit of editorial input was to trim the range of energies given for baryonic matter (which Cline explicitly equates to nucleons + electrons). He gives 106 to 109eV. I reduced this to an "average" 108eV, because this OOM gives internally consistency with the rest of his table entries. I tried to remain consistent with this bit of blurring by referring to "baryonic matter" rather than "baryons", but I should have been more explicit about what I was up to.

For neutrinos, Cline gives a mass-energy < 1eV, and a count of 1087. Total contribution to the mass-energy of the Universe is 0.3% according to his table, which by consistency with his photon figures seems to imply a working figure for the neutrino mass of 0.006eV: I've no idea where that comes from.

Grant Hutchison

Ken G
2006-Sep-03, 01:29 AM
Maybe it's the current best guess, I really don't know either. But the bottom line appears to be that photon energy is a thousand times less than the normal matter, and neutrinos might be about ten times less or so. What's also interesting is that the 10^87 photons and 10^78 protons haven't changed since the epoch of recombination, but the photons at that time were about 1,000 times more energetic than now. Thus at that time you had 10^87 photons at about 10^-1 eV and 10^77 protons at 10^9 eV, so the photon energy couldn't have been too much different from the proton energy at that point, curiously.

SirThoreth
2006-Sep-03, 01:50 AM
To answer drhex's question: photons are indeed included in the ordinary matter of the Universe. 'Matter' does not just include things that have mass.

In other words, to see if I understand correctly, when they say 4-5% "ordinary" matter, cosmologists are referring to baryonic matter (protons, neutrons, electrons, and their subatomic components), as well as photons - ie., that 4-5% is comprised of the "stuff we can see" so to speak, correct?

Ken G
2006-Sep-03, 01:54 AM
Yes, it's protons, photons, and neutrinos. There are probably no other players of note, in any normal epoch that has been observed. Then of course there is likely also dark matter and dark energy.

drhex
2009-Feb-16, 02:05 PM
Bringing up this thread from the dead again...

Isn't it true that particles of matter can be viewed as waves too?
Shouldn't those waves also be redshifted as the universe expands, meaning that photons and matter get diluted equally quickly as time passes?

Jeff Root
2009-Feb-16, 03:30 PM
The cosmic background radiation and starlight are redshifted relative
to the matter they are passing by, due to their having originated from
matter in a distant part of the Universe which was, at the time the light
was emitted, moving relative to our current position. That doesn't apply
to atoms and ions. It should apply to neutrinos if they are massless.

-- Jeff, in Minneapolis

Amber Robot
2009-Feb-16, 05:37 PM
There was a time, very early in the universe, when photons dominated the energy budget, but expansion is hard on photon energy.

Where did the energy of the photons go?

Grey
2009-Feb-16, 06:02 PM
Bringing up this thread from the dead again...

Isn't it true that particles of matter can be viewed as waves too?
Shouldn't those waves also be redshifted as the universe expands, meaning that photons and matter get diluted equally quickly as time passes?The answer is yes, sort of. :) The wavelength of a particle is related to its momentum, and so as the wavelength increases, it loses momentum. So that means that the amount of energy lost this way ends up being related to the kinetic energy, not the energy tied up as rest mass for any given particle. For photons, none of their energy is rest energy, so they are strongly affected by this. Highly relativistic particles, which have a large non-rest-mass contribution to their energy, would indeed lose energy due to the expansion of the universe in a manner comparable to photons. Neutrinos would fit into this category. On the other hand, typical slow moving particles have almost all of their energy as rest mass, and that isn't affected by cosmological redshift. No matter how much energy a particle loses due to cosmological redshift, it never falls below its rest mass.

Jeff Root
2009-Feb-16, 06:03 PM
There was a time, very early in the universe, when photons dominated
the energy budget, but expansion is hard on photon energy.
Where did the energy of the photons go?
They don't actually lose energy. Energy is relative. Relative to
the place where they started, they actually GAIN energy!

A photon of green light emitted ten billion years ago had a certain
energy relative to the matter it was emitted from. Its energy relative
to matter a billion light-years away from it was much less, because
the matter at the two locations was moving apart. By the time the
light reaches the second bunch of matter, it has traveled a distance of
ten billion light-years, and its energy relative to the matter around it
has reduced so much that it is now red. But its energy relative to the
matter it was emitted from has increased. It is now blue light relative
to that now-distant matter. But that now-distant matter can't see
the blue light that left it as green light ten billion years ago.

-- Jeff, in Minneapolis

Grey
2009-Feb-16, 06:11 PM
Where did the energy of the photons go?It's a trickier question than you might think. To talk about anything involving cosmological expansion (such as the change in a photon's energy because of that process), we have to use general relativity. And it turns out that conservation of energy is a little tricky in general relativity. It holds perfectly fine for any small region of space, but it turns out that even defining the total energy of a large region of space isn't as easy as it seems like it should be. It comes down to the fact that the energy of a system can only be defined with respect to some asymptotically flat reference frame, and in general relativity, there is simply no such thing as a globally valid asymptotically flat reference frame. Without being able to uniqely define the amount of energy in a given system, it doesn't really make much sense to talk about whether energy is lost or gained. Here (http://math.ucr.edu/home/baez/physics/Relativity/GR/energy_gr.html)'s a more detailed discussion.

GOURDHEAD
2009-Feb-16, 07:24 PM
If an infra-red energy-level photon, in its local reference frame, should encounter an object made from hadrons such that their relative speeds were such that, as "observed" by the hadronic (non-conscious) object, the photon were blue-shifted into the gamma range with sufficient energy to support pair formation, would pair formation occur? Can lower energy photons decay into neutrinos and anti-neutrinos? Any good references to these kinds of phenomena would be appreciated. If we succeed in achieving interstellar travel with ships traveling above 0.999c, we will likely need to feel that we have sound answers to these types of questions--let's hope I'm one of the few who doesn't already know the answers.

Grey
2009-Feb-16, 08:14 PM
If an infra-red energy-level photon, in its local reference frame, should encounter an object made from hadrons such that their relative speeds were such that, as "observed" by the hadronic (non-conscious) object, the photon were blue-shifted into the gamma range with sufficient energy to support pair formation, would pair formation occur?Remember that there's really no such thing as the reference frame of a photon. However, if you mean that we set our reference frame relative to some object with a small proper motion relative to the Hubble flow, and we see a low energy (relative to us) photon encounter some high energy (relative to us) particle, yes, pair production is one possible event. From our perspective, the energy comes from the kinetic energy of the hadron. In the rest frame of the hadron, the photon is highly energetic, and we could say it supplies the energy for the pair production. Since the energy of a given particle is frame dependent, either view is equally valid.


Can lower energy photons decay into neutrinos and anti-neutrinos?Not really. photons are carriers of the electromagnetic force, and neutrinos don't involve themselves in electromagnetic interactions. It's possible in principle for a photon to interact with some other particle or collection of particles, raising the energy to a higher state, and then have that colleciton of particles return to a lower state thropugh a process that emits neutrinos or antineutrinos. At very high energy levels, the electromagnetic and weak forces merge (on the order of 100 GeV, corresponding to a temperature of 1015 K), and I think this interaction becomes possible in principle, at least.


If we succeed in achieving interstellar travel with ships traveling above 0.999c, we will likely need to feel that we have sound answers to these types of questions--let's hope I'm one of the few who doesn't already know the answers.Sadly, we're a long, long way off from being able to travel at anything even vaguely approaching relativistic speeds. The fastest thing we've launched to date is New Horizons, with a speed of about 0.0054% of lightspeed.

grant hutchison
2009-Feb-16, 08:29 PM
No matter how much energy a particle loses due to cosmological redshift, it never falls below its rest mass.Which, if I'm thinking about this properly (and please correct me if I'm wrong), is directly related to the fact that if you haul an object out of the Hubble flow and then leave it to its own devices in intergalactic space, it will eventually rejoin the Hubble flow elsewhere (at least asymptotically). So that's an example of an object losing energy to cosmic expansion and eventually settling down to manifest only its rest mass relative to the expanding metric.

Grant Hutchison

Jeff Root
2009-Feb-16, 09:38 PM
if you haul an object out of the Hubble flow and then leave it to its
own devices in intergalactic space, it will eventually rejoin the Hubble
flow elsewhere (at least asymptotically). So that's an example of an
object losing energy to cosmic expansion and eventually settling down
to manifest only its rest mass relative to the expanding metric.
But the object doesn't lose any energy or settle down.

If you are at the back of a group of 1,000 runners, and you run
faster than 950 of them, you will move forward through the group,
passing runners as you go, until you catch up to the 49th runner
from the front, who you are unable to pass because you both run
at the same speed.

Oh, okay, you lost a lot of energy doing that and you're going to
have to settle down...

-- Jeff, in Minneapolis

grant hutchison
2009-Feb-16, 10:14 PM
But the object doesn't lose any energy or settle down.Take a look at Davis et al.'s Solutions to the tethered galaxy problem in an expanding Universe and the observation of receding blueshifted objects (http://arxiv.org/abs/astro-ph/0104349).
According to Davis et al. (some people disagree), objects with peculiar motion relative to the metric asymptotically lose that motion and join the Hubble flow. They lose energy relative to the metric in a way analogous to the cosmological redshift, except (as Grey says) they settle asymptotically towards their rest mass energy, rather than towards zero energy.

Grant Hutchison

Spaceman Spiff
2009-Feb-17, 03:13 AM
It's a trickier question than you might think. To talk about anything involving cosmological expansion (such as the change in a photon's energy because of that process), we have to use general relativity. And it turns out that conservation of energy is a little tricky in general relativity. It holds perfectly fine for any small region of space, but it turns out that even defining the total energy of a large region of space isn't as easy as it seems like it should be. It comes down to the fact that the energy of a system can only be defined with respect to some asymptotically flat reference frame, and in general relativity, there is simply no such thing as a globally valid asymptotically flat reference frame. Without being able to uniqely define the amount of energy in a given system, it doesn't really make much sense to talk about whether energy is lost or gained. Here (http://math.ucr.edu/home/baez/physics/Relativity/GR/energy_gr.html)'s a more detailed discussion.

Grey -- Back over at the "Olbers' Paradox" thread, a few days ago I gave a summary (http://www.bautforum.com/astronomy/80909-olbers-paradox-5.html#post1433322) of that Baez article you link to. Is that about right? Feel free to hack at the less precise or inaccurate wording. Man, I learn a lot from you.

Spaceman Spiff
2009-Feb-17, 03:16 AM
For neutrinos, Cline gives a mass-energy < 1eV, and a count of 1087. Total contribution to the mass-energy of the Universe is 0.3% according to his table, which by consistency with his photon figures seems to imply a working figure for the neutrino mass of 0.006eV: I've no idea where that comes from.

Grant Hutchison

This agrees with the default masses of the 3 neutrino types that appear in Ned Wright's cosmology calculator (http://www.astro.ucla.edu/%7Ewright/ACC.html) (the advanced version): sum = 0.0059 eV/c^2. So they must be working off similar estimates...

Spaceman Spiff
2009-Feb-17, 03:23 AM
They don't actually lose energy. Energy is relative. Relative to
the place where they started, they actually GAIN energy!

A photon of green light emitted ten billion years ago had a certain
energy relative to the matter it was emitted from. Its energy relative
to matter a billion light-years away from it was much less, because
the matter at the two locations was moving apart. By the time the
light reaches the second bunch of matter, it has traveled a distance of
ten billion light-years, and its energy relative to the matter around it
has reduced so much that it is now red. But its energy relative to the
matter it was emitted from has increased. It is now blue light relative
to that now-distant matter. But that now-distant matter can't see
the blue light that left it as green light ten billion years ago.

-- Jeff, in Minneapolis

Why do you say this? :confused:
In what way does an astronomer in that distant (in space-time) galaxy view the universe differently than one does here and now?

Jeff Root
2009-Feb-17, 08:29 AM
But its energy relative to the
matter it was emitted from has increased. It is now blue light relative
to that now-distant matter. But that now-distant matter can't see
the blue light that left it as green light ten billion years ago.
Why do you say this? :confused:
It seemed like a good idea at the time?



In what way does an astronomer in that distant (in space-time)
galaxy view the universe differently than one does here and now?
I'm not going to answer that because, even if I'm right that the
energy increases relative to the distant matter, it isn't relevant,
for the reason given in my quoted final sentence: Nobody can see
the increase in energy. So it has no practical consequence.

However, maybe I missed something and am wrong about the
energy increase. Maybe it stays the same relative to the matter
it was emitted from. That actually makes more sense to me now.

Can I change my plea?

-- Jeff, in Minneapolis