Missing Matter, Missing Energy?

• OB 50
In summary, the energy density of radiation drops off with the expansion of the universe, meaning that at the current time radiation density is a completely inconsequential fraction of the total energy density. However, we do know the energy density of radiation by observing the cosmic microwave background, which makes up most of the energy density of the universe.

OB 50

I constantly see various figures tossed around stating what the total mass contained in the universe should be versus what it is observed to be. Apparently, quite a bit of it is missing.

Maybe it's because my exposure to this problem is through articles aimed at my level of understanding (the well-informed layman), but there seems to be an aspect of the problem that I've never seen explicitly addressed.

Energy is equivalent to mass, and vice versa. Most matter in the universe radiates energy in the form of heat or light. The easiest example of this would be a star like our own sun. It has a finite life span during which it will emit light and heat and various other forms of EM radiation.

The vast majority of this energy will never hit or interact with anything. It is simply sent out into the void.

This didn't really bother me until I started thinking about the Cosmic Microwave Background radiation. The radiation we observe from this source is billions of years old. That energy has been in transit for billions and billions of years before reaching us, and most of it whizzes right on by. It's pretty faint, but when you consider that every conceivable point within the observable universe is capable of observing that very same energy, it seems like it would be a staggeringly huge amount.

That's just "right now"; what about the effect over time? Would it be a stretch to say that the entire history of the universe exists as EM radiation in transit, aside from what is absorbed or interacted with directly by other matter?

I guess my question boils down to this:

Is the sum total of all energy radiated by the matter in the universe throughout history accounted for in the calculations that determine how much matter should be present?

I had a lot of questions about the same subject, but I got ahead of myself and asked to many questions at once as well as giving speculative, personal theory. From what I got out of it, its all hypothetical at the moment. With some new discoveries like neutrinos that do have mass, but are created by radioactive decay or nuclear reactions in nuclear reactors, or when cosmic rays hit atoms. I am very interested in this as well. For no other reason than just being curious.

OB 50 said:
I guess my question boils down to this:

Is the sum total of all energy radiated by the matter in the universe throughout history accounted for in the calculations that determine how much matter should be present?
Well, the problem with this idea is that the energy density of radiation drops off with the expansion of the universe as 1/a^4. As a result, at the current time radiation density is a completely inconsequential fraction of the total energy density.

Chalnoth said:
Well, the problem with this idea is that the energy density of radiation drops off with the expansion of the universe as 1/a^4. As a result, at the current time radiation density is a completely inconsequential fraction of the total energy density.

Is this generally agreed and doesn't it presuppose that the total amount of radiation energy is known.Like the others I am interested in this question and I will be grateful if you can elaborate.

Is this generally agreed
Yes. There really is no question as to the validity of how the energy density of radiation scales with the expansion of the universe. Basically, it's just how matter behaves when it moves at or near the speed of light: anything that does that scales in energy density with expansion as 1/a^4.

and doesn't it presuppose that the total amount of radiation energy is known.
No. But we do know it, to extremely high accuracy, by observations of the cosmic microwave background (which makes up most of the energy density of the universe in radiation...and it's pretty small today compared to the energy density in normal and dark matter).

Chalnoth said:
Yes. There really is no question as to the validity of how the energy density of radiation scales with the expansion of the universe. Basically, it's just how matter behaves when it moves at or near the speed of light: anything that does that scales in energy density with expansion as 1/a^4.

No. But we do know it, to extremely high accuracy, by observations of the cosmic microwave background (which makes up most of the energy density of the universe in radiation...and it's pretty small today compared to the energy density in normal and dark matter).

Thankyou for your answer but I don't understand your second point.The microwave radiation is detectible but could there be other radiation energy that has not yet,or perhaps never will be, detected?Or is it that we are able to estimate the energy carried by other wavelengths by applying something like the theory of black body radiation to the data we have on microwaves?

Thankyou for your answer but I don't understand your second point.The microwave radiation is detectible but could there be other radiation energy that has not yet,or perhaps never will be, detected?Or is it that we are able to estimate the energy carried by other wavelengths by applying something like the theory of black body radiation to the data we have on microwaves?
The point is that there really is no reason to believe that any other emitted radiation could be anywhere near as large in total energy density. And even if we were wrong, and the total energy density of all of the radiation emitted was a hundred times that of the CMB, the total amount of radiation still would be smaller than the matter energy density.

Chalnoth said:
Yes. There really is no question as to the validity of how the energy density of radiation scales with the expansion of the universe. Basically, it's just how matter behaves when it moves at or near the speed of light: anything that does that scales in energy density with expansion as 1/a^4.

No. But we do know it, to extremely high accuracy, by observations of the cosmic microwave background (which makes up most of the energy density of the universe in radiation...and it's pretty small today compared to the energy density in normal and dark matter).

I don't disagree with anything you've said, and I'm aware of how energy density scales over distance and time. What I'm trying to understand is how all the pieces fit together when viewed as a whole, from a historical perspective.

For instance, at one point in the universe's very early history, solid matter had not yet condensed from energy. Would it be correct to say that the radiation energy density at this point made up nearly 100%? The universe had to "cool" from that state in order for matter to form, so that energy had to be radiated somewhere. In the universe's pre-matter state, was any of it already "missing"? If not, then where along the continuum of change from that state to the present did it "go away"?

My fundamental concern doesn't necessarily lie with the methods used for calculating the answer to the question of "missing matter", but more along the lines of how did we frame the question in the first place?

It seems like there are too many variables to even make an educated guess. We don't know the shape or topography of the universe. We're limited to what we can see in our observable area of 14 billion LY or so, so who knows what lies beyond that. We don't even understand what gravity is or exactly how it works even at a relatively short distance (Voyager probe trajctory, dwarf galaxy orbits, etc.). Those seem like they would all be essential elements for arriving at an accurate answer, yet they are all in some sense unknowns.

Have you ever had someone try to tell you that it's aerodynamically impossible for bumblebees to fly? It's absurd. Bumblebees DO fly, so obviously the fault lies with an inadequate understanding of aerodynamics. This is the same gut reaction I get from the missing matter/dark energy concept.

Obviously these other alternatives are being investigated. It is still an open question, for instance, whether or not the observed acceleration of the universe is being caused by some modification of gravity on very large scales. Most theorists think it very weird and unnatural that such a modification would be the case (as we expect the modifications to GR to be quantum in nature, modifications that would become apparent at short distances, not large ones). However, the possibility is still being investigated, and current and future measures of the growth of structure in the universe (galaxies, galaxy clusters, etc.) should be able to distinguish between dark energy and at least some forms of modified gravity.

Other possibilities have also been put forward in the past, and been struck down one by one. For example, the initial measurements of the acceleration of the universe basically came down to distant supernovae being slightly dimmer than we would have otherwise expected. So, we investigated a number of theories that could cause a dimming to occur for light traveling very long distances: this was largely struck down because exactly as the LCDM model predicts, when you go far enough back the universe's expansion was decelerating.

Then there was the possibility that perhaps stuff outside our horizon that we can't see might be influencing our universe, causing the acceleration with no weird dark energy or modified gravity. Well, it turned out that this can't be the case either: the effects of stuff outside the horizon just manifests itself as different initial conditions for the stuff inside, which means that it can't cause an apparent acceleration.

Others put forward the possibility that perhaps we weren't dealing with the impact of inhomogeneities properly: the universe isn't, after all, perfectly smooth as the Friedman equations assume (the same equations used to estimate the contents of the universe). So maybe, in dealing more accurately with how the universe's density varies from place to place, the acceleration might be a natural consequence. Well, this also was investigated carefully and, as it turns out, no, it doesn't happen. The problem here is that you need a truly absurd distribution of matter (one that directly contradicts observations) to get an apparent acceleration like we observe.

So yeah, right now it's basically down to either modified gravity or dark energy. We just use "dark energy" because it's short and sweet, though to be perfectly accurate we should include both.