# Why is the CMB not simply travelling parallel to us? [closed]

When we look to the distant farthest reaches of the universe we see light that was emitted at the big bang 14 billion years ago. But the universe was tiny back then so that light, which is only reaching us now, was emitted pretty much right next to us (in galactic terms).

So it's been traveling for 14 billion years at the speed of light and is only reaching us right now, despite only having set off from right next door.

The standard explanation of how it's covered such a tiny distance begins with "expansion", which means the light has been swimming against the tide effectively and since that doesn't fit properly, it's augmented by inventing inflation which is basically a huge rush of Hubbly expansion at the outset, to increase the distance away from us before it began swimming towards us in accordance with Hubble's law which we now observe.

If we instead explain how it's covered such a tiny distance by saying that we are all departing the big bang at or near the speed of light because we are actually for example something like Hawking Radiation, then we don't need "Expansion" do we? Because the CMB is just the light that was emitted near parallel to us and has been traveling through space next to us all this time. So it's quite natural that it is only now reaching us. And then we don't need to invent inflation, do we, to make Hubble's law and the Big Bang fit?

And since it's been traveling so close to parallel all this time it seems natural that it should have such a low level of energy - unlike light emitted right next to us recently.

And then we have a natural reason for mass-energy equivalence don't we, because everything's moving at the speed of light anyway.

And then we have a natural reason for quantum theory don't we because if everything around us is moving nearly parallel at the speed of light then only its direction can change, not its speed so we would expect to find that if we impose a speed vector on objects which only have direction, their degrees of freedom will be over-specified and we won't be able to define their parameters all at once because they don't really exist.

And then it becomes obvious that "perceived speed" is simply an aggregation of how parallel the component parts of something are, so simple objects containing minimal component parts must travel at the speed light while compound things can move more slowly because they have component parts moving in different, changing directions around each other and their overall motion is the combination of the parts.

And then that would predict that a photon can't age.

And then everything seems to make a lot more sense because when we look out at the "Big Bang" in distant space and see it receding at the speed of light, well that's what we know it to be doing anyway so whoopdedoo who cares.

And because the universe is now just a set of paths in 4-dimensional space with no "speed", it should be described by a rotation group SO(3,1) and oh hang on haven't I seen that somewhere before, like the set of transformations in relativity?

But then we would all have to be made of light. So how do we design an experiment in which we bounce beams of appropriately formed light off each other itself at exceptionally low angles of incidence in order to prove that they do, to the surprise of almost the entire physics community, bounce off each other?

And how did we get in such a pickle inventing all this inflation and dark matter?

## closed as too broad by James K, Hohmannfan, Rob Jeffries, Andy, adrianmcmenaminJul 2 '16 at 17:32

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• Who said the Universe was tiny at the time of recombination? Most think it was infinite, and even if finite still not tiny at 380,000 years after the BB – Conrad Turner Jun 25 '16 at 15:46
• The part of it within our event horizon was tiny in galactic terms. – user334732 Jun 25 '16 at 17:28
• Take care not to confuse inflation (stopped after an octillioth of a second) with expansion (still going) – James K Jun 25 '16 at 19:23
• @RobertFrost radius of ~20-40 Mly – Conrad Turner Jun 25 '16 at 20:11
• @JamesKilfiger I did. – user334732 Jun 25 '16 at 22:28

Some of your questions seem to be genuine enough, so I'll address those:

When the CMB was emitted, it was emitted everywhere in the Universe, in all directions. That means that we see it at all times, but the part that we see at a given time comes from an ever-increasing distance. Today that distance is roughly 47 billion lightyears (Gly). It is also redshifted, today by a factor of $z \simeq 1100$. That means that when it was emitted, the Universe was a factor 1100 smaller (in all three directions). Hence, that particular part of the Universe that emitted the light that we see today was $47\,\mathrm{Gly} \,/\, 1100 \simeq 43 \,\mathrm{Mly}$.

Since the CMB is observed from all directions, only light from small patches on the sky can be regarded as being parallel. Obviously light from the left of you and above you is not parallel.

The CMB hasn't really been "right on our heels" since it was emitted, any more than, say, the cars overtaking you when you riding your bike are right on your heels. You're just constantly being overtaken by new CMB photons. The part we see today was actually carried away from us by expansion for the first 4 billion years or so, before it overtook expansion and started reducing its distance from us.

Inflation, as James Kilfiger comments, is something completely unrelated, taking place when the Universe was some $10^{-33}$ s old, as opposed to the CMB which was emitted when the Universe was 380,000 years old.

• I'm familiar with the dominant paradigm. If it comes from an ever increasing distance then does that mean we're receding from the point where it was emitted? So we agree on that then. Red shift doesn't discriminate between our theories; it's predicted by both. Obviously if you are moving along a path and light starts all around you and travels nearly parallel with you, when it finally arrives it appears to come from all around you despite being parallel since you only perceive the relative motion. – user334732 Jun 25 '16 at 22:43
• Inflation is a way of balancing the books when you find the bottom lines don't tally up. – user334732 Jun 25 '16 at 22:45
• Space as the surface of an expanding 3-sphere is stronger under Occam's razor than the current paradigm. It requires fewer assumptions. – user334732 Jun 25 '16 at 22:52
• @RobertFrost But that's not the case. That's the result of a visualization that can be poorly extended to create wrong implications. – HDE 226868 Jun 25 '16 at 23:17
• The CMB coming from an ever-increasing distance is not (only) a consequence of expansion. You'd have the same in a non-expanding universe, simply because light emitted farther away takes a longer time reaching you. As for the traveling-parallel-to-us hypothesis… that seems to imply that we're traveling through space. We're not, at least not very fast, only at a few 100 km/s. Maybe you think that we're moving away from a Big Bang? We're not. – pela Jun 25 '16 at 23:19