Was Energy in the Expanding Universe Traveling at the Speed of Light?

[NeedBranes]

When our universe first started to expand, is it safe to assume that the energy inside of the expanding universe was traveling at the speed of light?

 

[Chronos]

No more so than assuming space was expanding at the speed of light. The separation of pockets of energy in the primodial universe would have increased proportionate with inflation.

 

[marcus]

When our universe first started to expand, is it safe to assume that the energy inside of the expanding universe was traveling at the speed of light?

Sometimes people talk about things "traveling" when they mean things getting farther apart.

In the mainstream picture of the early U, distances between things are typically growing thousands of times faster than speed of light.
The way it's normally modeled, with distances defined the way astronomers usually do when discussing expansion, distances between things or places would typically be increasing much much faster than c.

But any given bit of light would, in a limited sense relative to its immediate surroundings, be "traveling" at speed c, because that is what light does. Only that would not have very much to do with the expansion of distances that was going on at the same time. That is just an insignificant local motion of that bit of light.

So for the question to be clear you have to say explicitly what you mean by "traveling".
Are you talking about the expansion process? If so, then what you say is NOT safe to assume. Or are you talking about the comparatively tiny local motion of an individual photon of light at speed c (insignificant relative to the overall expansion process). In which case it IS safe to assume---the photon is traveling at speed c, as you say.

 

[NeedBranes]

I asked if the energy right after the expansion of space moved at the speed if light only because I assumed that this energy would be massless.

 

[Naty1]

You posed a very different issue in post #4

..energy right after the expansion of space

than your original post

When our universe first started to expand...

Do you see the vast difference?

The prior posts assumed you were talking about the initial INFLATIONARY ...when
everything expanded at greater than the speed of light.

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


Last I read, a false vacuum [ false meaning temporarily at an elevated vacuum energy density] according to general relativity leads to gravitational repulsion. So that vacuum energy is what powered the very early expansion.

 

[NeedBranes]

The energy (or matter) of our universe, which is not presumed to have negative pressure or of any relation to inflationary energy, can all be traced back to a point in time, in our universe, in which it was massless. So it would be moving at the speed of light. Right?

 

[Drakkith]

The energy (or matter) of our universe, which is not presumed to have negative pressure or of any relation to inflationary energy, can all be traced back to a point in time, in our universe, in which it was massless. So it would be moving at the speed of light. Right?

It depends on what you mean by "energy moving". Energy is a quantity, but not a physical object. What do you mean by asking what speed it moves at?

 

[osxraider]

We recently touched upon a topic/calculation in my Astrophysics class where things were indeed moving away from each other at the speed of light. Unfortunately, I just returned my rental Astrophysics book.

I would say that the Universe expanded ridiculously fast (just like galaxies receding) but they were not exceeding the velocity of light but that space being created between them was causing them to move apart faster than light that might be able to travel between them However, in the early Universe, light could only begin traveling after the Universe became transparent enough for photons to travel long mean free paths.

So no, I don't think particles were ever traveling faster than light speed but yes, it expanded much faster than light could travel between the initial size and size after a definite interval of time.

Also, this is supposed to be valid only after the Planck time. Before this and right after "time=0", we can't really tell what was happening in terms of today's physical laws.

 

[Drakkith]

I would say that the Universe expanded ridiculously fast (just like galaxies receding) but they were not exceeding the velocity of light but that space being created between them was causing them to move apart faster than light that might be able to travel between them However, in the early Universe, light could only begin traveling after the Universe became transparent enough for photons to travel long mean free paths.

How is this expansion somehow different from current expansion? I would think that since expansion causes galaxies to recede from each other at an ever increasing velocity as you increase the distance, the same would apply to the early universe. This would cause objects a certain distance away from any observer to recede faster than the speed of light.

 

[osxraider]

I think the rate was different. Isn't that why inflation was put in? I believe that the rate has slowed?

I was confused myself but apparently, inflation (the ridiculously crazy expansion that flattened the Universe into uniformity said to be caused due to "reverse gravity") is a little different that the current expansion due to dark energy.

 

[Drakkith]

I think the rate was different. Isn't that why inflation was put in? I believe that the rate has slowed?

I was confused myself but apparently, inflation (the ridiculously crazy expansion that flattened the Universe into uniformity said to be caused due to "reverse gravity") is a little different that the current expansion due to dark energy.

But as you said, expansion is a rate. Not a velocity. Wouldn't that require that the recession velocity increase as distance increases? Even in the early universe?

 

[osxraider]

Yes, you are absolutely correct! but in an early universe, there was a point at which the distance might not have been big enough to cause a recession velocity that was faster than the speed of light. KEYWORD: MAY OR MAY NOT

 

[Drakkith]

Yes, you are absolutely correct! but in an early universe, there was a point at which the distance might not have been big enough to cause a recession velocity that was faster than the speed of light. KEYWORD: MAY OR MAY NOT

Maybe. I prefer to talk about expansion as if we live in an infinite universe. Otherwise confusing issues like this happen if we don't.

 

[osxraider]

That is very dangerous because it could imply infinite recession velocity. That means in a Universe that is been around for a finite amount of time, something has managed to appear to go away from us at infinite speed?

 

[Drakkith]

That is very dangerous because it could imply infinite recession velocity. That means in a Universe that is been around for a finite amount of time, something has managed to appear to go away from us at infinite speed?

Take any distance and you will get a finite value for its recession velocity.

 

[Naty1]

Quote by NeedBranes

The energy (or matter) of our universe, which is not presumed to have negative pressure or of any relation to inflationary energy, can all be traced back to a point in time, in our universe, in which it was massless. So it would be moving at the speed of light. Right?

Richard Feynman,

It is important to realize that in physics today, we have no knowledge what energy is. We do not have a picture that energy comes in little blobs of a definite amount.

So it's difficult to provide a one line answer. Besides, I don't know what 'it' is in your question.

One way to think of energy is as an element of our field theories. So for example, electromagnetic energy is carried by massless photons, and gravitational energy by massless gravitons. Today these forces carry causal effects at the speed of light according to relativity theory. During inflationary expansion early in the universe, whatever energy forms existed seem to have expanded at greater than light speed.
I believe nuclear forces and the associated energy, like binding energy, also move at
lightspeed as reflected in relativistiv quantum theory of the Standard Model of particle physics.

Today, when we calculate very distant motions we can also arrive at superluminal recession speeds. For example the Hubble radius distance at which we calculate massive objects receding at speed 'c'. But that results from our cosmological model metric [distance measure] and it's exact meaning can be easily misunderstood. For example, it does not mean matter and energy are moving faster than light locally.

 

[Naty1]

Inflation:

The early inflation seems to be different from inflation today; besides it being much faster than today. Here are brief snippets from THE ROAD TO REALITY by Roger Penrose, pages section 28.4. I am paraphrasing here:

The commonly described history of the universe begins with conventional causal expansion. [This means, for example, that thermodynamic energy had enough time to create rather uniform temperatures...to 'communicate' causal effects.] Then from about 10^-35 second to about 10^-32 seconds an arbitrary scalar field is introduced [sometimes referred to a 'Higgs field']. 'Some models require more than one inflationary phase, in which case there would have to be a different scalar field for each'.
...'During the inflation stage we have a region of a false vacuum which represents a quantum mechanical phase transition to a vacuum different from the one we are familiar with today'...Lambda [the cosmological constant of expansion] was about 10¹¹⁰ times it's current density. This transition is the 'slow roll' version
described in Wikipedia

http://en.wikipedia.org/wiki/Cosmic_inflation#Slow-roll_inflation

and the initial inflation ends when the brief 'roll' of the phase transition completes. [This
slow roll allows the universe time to achieve observed characteristics.]

Penrose doesn't actually say causal connection is lost during inflation, but explains causal connection takes place before this brief inflationary period. Stuff moves faster than light.

Note:
[Penrose seems to think such an arbitrary introduction of a field to 'fix' problems of the 'old model' 'casts doubt on the entire idea' and 'few will be as negative as I am' but he acknowledges 'inflationary cosmology, as described, has become a major part of the body of modern cosmological thinking'..

 

[NeedBranes]

Inflationary energy and dark energy are seen to be different from one another. Could inflationary energy and dark energy be two aspects of the same energy? Only viewed as different from one another because inflationary energy had less "space" to expand than dark energy has today, or is this just not how we view the two?

 

[Naty1]

Could inflationary energy and dark energy be two aspects of the same energy?

yes and no. yes they were likely once related, but not currently. Here is a quick explanation of cosmological energy evolution...sorry, did not record the source...

Keep in mind EVERYTHING observed today, matter, energy, the cosmos, you,me, ice cream, evolved from a single entity...the 'Big Bang'...and as time passes, the forms change. For example, it is believed more and more entropy is being absorbed by black holes and eventually they will be all that is left in the universe [Princeton physicist Paul Steinhardt explained this on radio last night].These few paragraphs provide a brief overview of how energy is believed to evolve from beginning to end.

A most basic fact is that the universe evolves: We do not live in a static cosmos. Total energy in the universe is constant, but its form is steadily evolving from a low entropy to a high entropy form. All forms of energy, such as matter, fission, fusion, and kinetic and potential energy are precisely offset by the negative gravitational potential of everything. All energy is contained either in matter or force fields and the theoretical Higgs field which imparts mass to certain particles (as protons, neutrons and electrons). The universe ... began from a very high energy, high temperature, unstable state. It evolves towards a more stable lower temperature state and might eventually reach absolute zero in the big bang model. Inflationary energy created exponential expansion just after the big bang and such energy decays into radiation via quantum mechanical processes. ...

In the beginning it’s believed most energy was in the form of heat radiation and inflationary (vacuum) energy from which space, time, and matter evolved. As the universe expands, the total vacuum energy increases and its repulsive nature causes space to expand even faster…in a run away exponential expansion. In the big bang model, dark energy [the cosmological constant] powers an accelerated expansion after 9B years as it overpowers the gravitational attraction of matter.

Matter energy density decreases as volume grows and causes matter to spread out; radiation energy density not only decreases but its wavelength is stretched further by expansion depleting its energy faster, so radiation density falls faster than the energy density of matter. Dark matter energy density remains nearly constant. Dark matter dominated the past; dark energy will dominate the future. Dark matter has nearly three times the energy density of dark energy.

and just in case you were wondering, Chronos recently posted this:

Dark matter and dark energy are related by name only, they are distinctively different. We infer the existence of dark matter by its gravitational effects. We have no idea about the properties of DM particles. It does, however, appear to be 'cold' - i.e., generally not moving at relativistic speeds. The existence of DE is inferred strictly from its effect on expansion of the universe.

 

[Naty1]

Also see my recent post #8 here:

https://www.physicsforums.com/showthread.php?t=660269