Jump to content

Stargazers Lounge Uses Cookies

Like most websites, SGL uses cookies in order to deliver a secure, personalised service, to provide social media functions and to analyse our traffic. Continued use of SGL indicates your acceptance of our cookie policy.

sgl_imaging_challenge_banner_30_second_exp_2_winners.thumb.jpg.b5430b40547c40d344fd4493776ab99f.jpg

MJP

Cosmic Microwave Background

Recommended Posts

I rather wonder why the cosmic background radiation is still visible. My thinking is that this burst of light happened early in the history of the universe but was a "one off"event.

Now if we imagine I shine a torch out towards space for an hour, and then turn it off, then someone at a point way out in space would see that light (let's assume it is a very powerful torch!) then one hour later the light would go out. The light wave will have passed them and could still be seen from a still more distant point but again only for one hour( although obviously not the same hour).

Therefore why do we continue to see the cosmic background radiation. I would expect the burst of light (at microwave wave lengths) to have passed us or maybe we are in a favourable position and it will cease to be visible at some point in the future.

Or maybe I just don't understand something basic!

Mike

Share this post


Link to post
Share on other sites

I would guess that you are thinking about it in the wrong terms. We are, and the CMB is, part of the same event. It isn't something that happened 100 light years away and so is over and done with but is part of the fabric of the universe, as are we, and so we see the CMB as an ongoing event, which has now cooled to its present level. That is how I see it anyway.

Edited by sologuitarist61

Share this post


Link to post
Share on other sites

The CMB happened everywhere. Before the event it was too hot for atoms to form, so lots of free electrons around scattering light.

Then it cooled enough for the atoms and nuclei to combine and thats when we see the surface of last scattering. But it happened everywhere. Here, across the galaxy, and across the universe, so there is lots of it to go around!

Its like everyone in the world turned on their torches for a few seconds. We'd see the near ones, then the not so near ones, and so on.

Share this post


Link to post
Share on other sites

Dont forget that we are looking back in time, not only is it 13.odd billions years ago, its also 13.odd billion years of expansion away.

Everything happened everywhere at once and everything was and is and will be expanding / inflating from our perspective away from us as if we are in the middle.

Share this post


Link to post
Share on other sites

Afraid you have not really understood what the CMB is.

The Cosmic Microwave Background (CMB) radiation, is the faint glow of light that fills the universe, which is nearly uniform intensity in all directions that we look. It is the residual heat of creation. In other words it is the afterglow of the big bang. But you should not think of it in terms of a flash a light from the moment of creation like your torch analogy.

The CMB is the oldest light we can see. The farthest back both in time and space that we can look. This light set out on its journey approximately 14 billion years ago. It is a relic of the universe's infancy, a time when it was not the cold dark place we see now, but was instead a storm of radiation and elementary particles. When the universe was young, it was smaller, far hotter, and filled with a uniform white-hot fog of plasma. As the universe expanded, both the plasma and the radiation filling it grew cooler. When the universe cooled enough, protons and electrons could form neutral atoms. The consequence of the formation of these simple atoms like hydrogen is that they could no longer absorb the radiation, and the universe became transparent instead of being an opaque fog. This is known as the “recombination epoch,” and is the event shortly after which photons starting to travel freely through space rather than constantly scattering with electrons and protons in the plasma. This is known as “photon decoupling” and happened roughly 300,000 years after the big bang.

When this cosmic background light was released billions of years ago, the universe was still very hot, approximately 3,000 degrees. The photons that existed at the time of the photon decoupling have been propagating ever since, though growing fainter and less energetic, since the expansion of space (grown by a factor of 1,000 since then) causes their wavelength to increase over time. The wavelength of the light has stretched into the microwave part of the electromagnetic spectrum, and the CMB has cooled to its present-day temperature, about 2.73 degrees above absolute zero.

The CMB radiation we measure today comes from a spherical surface called the “surface of last scattering.” This is a set of locations in space at which the decoupling event occurred and at a point in time such that the photons from that distance have just reached us. Hopefully this will allow you why we continue to observe the CMB and will continue to do so, rather than the flash analogy of your torch. Though into the future it will continue to cool thanks to expansion of the universe.

  • Like 1

Share this post


Link to post
Share on other sites

Is 'light' the correct term? I thought the CMB was 'thermal radiation', which is different to 'light' - the word 'thermal' denotes heat, not light surely? :huh: :huh:

Share this post


Link to post
Share on other sites

Is 'light' the correct term? I thought the CMB was 'thermal radiation', which is different to 'light' - the word 'thermal' denotes heat, not light surely? :huh: :huh:

I use the term “light” interchangeably with “electromagnetic radiation” in an attempt to keep it in terms for the layperson, though perhaps I should have stated that to avoid further confusion. Thermal radiation is electromagnetic radiation generated by thermal motion of charged particles. While light in the context most people would understand would be the visible part of the spectrum that we are all aware of from about 400nm to 700nm wavelength, by using the term light, I hope indicates that what we are talking about is still a component of the wider electromagnetic spectrum.

Share this post


Link to post
Share on other sites

As above, the key flaw in the torch analogy is that it's light is from a point source and directional. Replace the torch with a spherical bulb and what happens to the analogy? It starts to resemble the CMB far more closely, though still very imperfectly.

It always tickles me that in science we stick with confusing terms arising from the accidental chronology of discovery. The useful term Electro Magnetic Radiation, which embraces gamma to radio via infra red, visible, ultra violet, etc etc, remains an obscure term in that it isn't in the lay vocabulary. I guess I think it should be but I like the retention of the pre-unifying vocabulary as well. Ho hum, language...

Afterglow of Creation, by Marcus Chown, is a splendid read. Recommended.

Olly

Edited by ollypenrice
  • Like 1

Share this post


Link to post
Share on other sites

Very interesting how the discovery process leads to things kind that and how it can then create confusion in the general public.

Always nice to come across analogies that can help those of us with a scientific background get to grips with a concept.

Share this post


Link to post
Share on other sites

Sorry should have said without, not with. Trouble with virtual keyboards on smart phones.

Share this post


Link to post
Share on other sites

Many thanks to all who replied. I appreciate your time and effort. Clearly I was looking at it in the wrong way.

Mike

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.


  • Recently Browsing   0 members

    No registered users viewing this page.

×
×
  • Create New...

Important Information

By using this site, you agree to our Terms of Use.