# Where is the cosmic microwave background? **URL:** \ **Category:** Factual Questions\ **Created:** [April 25, 2012, 5:34pm UTC](https://boards.straightdope.com/t/where-is-the-cosmic-microwave-background/619928 "2012-04-25T17:34:13Z")\ **Posts on this page:** 13\ **Page:** 1
**Author:** ![larryhughs](https://avatars.discourse-cdn.com/v4/letter/l/8baadc/32.png) [@larryhughs](https://boards.straightdope.com/u/larryhughs)\ **Post date:** [April 25, 2012, 5:34pm UTC](https://boards.straightdope.com/t/where-is-the-cosmic-microwave-background/619928/1 "2012-04-25T17:34:13Z")
Is it everywhere, every point in every direction? How do we view it? I am very confused. ---
**Author:** ![cmyk](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cmyk/32/3353_2.png) [@cmyk](https://boards.straightdope.com/u/cmyk)\ **Post date:** [April 25, 2012, 5:42pm UTC](https://boards.straightdope.com/t/where-is-the-cosmic-microwave-background/619928/2 "2012-04-25T17:42:35Z")
When the universe was only a few hundred thousand years old, the universe was still smaller, hotter and more dense than it is now. At this time, the universe was aglow with an almost even field of white-hot hydrogen plasma, making the entire universe essentially opaque with light. As the universe expanded and cooled, this plasma stopped glowing, and the universe became visible again, but since space is expanding, that glow has red-shifted into the microwave spectrum of the EM field. This radiation lies at the very boundary of the observable universe, and is as far back in time as we can look, since the universe was opaque with EM radiation prior to this. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [April 25, 2012, 5:47pm UTC](https://boards.straightdope.com/t/where-is-the-cosmic-microwave-background/619928/3 "2012-04-25T17:47:53Z")
There are a number of ways of answering this, so I’ll just give a full recap. Shortly after the Big Bang, everything in the Universe was hot, and emitting light. But everything was also opaque, so the emitted light didn’t get very far. Then, after a few hundred thousand years, the matter in the Universe cooled down enough that it rather abruptly became transparent. All that light that was previously just bouncing around was suddenly free, and went off in pretty much every direction from every point. It started off very energetic, but as the Universe expanded, it redshifted to a much lower frequency, and is now down to the microwave range. There was some of this light that originated here, like every other location. But that light is long gone, billions of lightyears away by now. The light that’s reaching us all originated at point billions of lightyears away from here, and happened to be going in the right direction to reach us (there’s plenty more light that was going in other directions, but we don’t see it). Some of the light that reaches our vicinity happens to hit a detector of some sort (a TV antenna will catch a little of it), some of it gets absorbed by things that aren’t detectors, and most of it just misses everything (space is mostly empty, after all) and flies right on past, possibly to interact with something else somewhere further down the line, but probably just flying on eternally through the void. So, to recap: There are photons of the CMB all over the place, going in all different directions. Every place was the origin of photons of the CMB, again, going in all directions. The photons of the CMB that are currently in any particular place all originated from far away, and happened to be going in the right direction to reach that point. ---
**Author:** ![engineer\_comp\_geek](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/engineer_comp_geek/32/504_2.png) [@engineer\_comp\_geek](https://boards.straightdope.com/u/engineer_comp_geek)\ **Post date:** [April 25, 2012, 5:51pm UTC](https://boards.straightdope.com/t/where-is-the-cosmic-microwave-background/619928/4 "2012-04-25T17:51:46Z")
> [@larryhughs](#): > > Is it everywhere, every point in every direction? Pretty much. It’s electromagnetic radiation, so in a sense it’s a lot like light from a flashlight, just lower in frequency. It’s not some solid kind of thing that’s just sitting at a point. But it is all around us, coming at us from every direction. Microwaves are radio waves. They are just a particular subset of radio waves. Cell phones and wireless internet are two common things that use microwave radio waves. > [@larryhughs](#): > > How do we view it? I am very confused. You “view” it with a radio antenna. If you point your radio antenna at different points in the sky, you can map out the intensity of the radio waves all across the sky and get a “picture” of what the background radiation looks like. Stars and other things emit radio waves as well, and you’ll see them in your “radio picture”. Astronomers use radio antennas to look at things in space all the time. Google “radio astronomy” for more details. Here is the wikipedia article on cosmic background radiation. While it is mostly uniform in every direction, there are some minor variations in it. The article includes a map which shows these variations. > **[Cosmic microwave background](https://en.wikipedia.org/wiki/Cosmic_microwave_background_radiation)** > > The cosmic microwave background (CMB, CMBR), or relic radiation, is microwave radiation that fills all space in the observable universe. With a standard optical telescope, the background space between stars and galaxies is almost completely dark. However, a sufficiently sensitive radio telescope detects a faint background glow that is almost uniform and is not associated with any star, galaxy, or other object. This glow is strongest in the microwave region of the electromagnetic spectrum. The acc... ---
**Author:** ![Voyager](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/voyager/32/133_2.png) [@Voyager](https://boards.straightdope.com/u/Voyager)\ **Post date:** [April 25, 2012, 7:41pm UTC](https://boards.straightdope.com/t/where-is-the-cosmic-microwave-background/619928/5 "2012-04-25T19:41:58Z")
That’s how Penzias and Wilson found it. a lot of long distance transmission is done using microwaves, and they were noticing noise from an unknown source. Using radio equipment at Bell Labs in Crawfords Corner, near Holmdel NJ they tried to pinpoint the source - and found was everywhere. They might have gotten Nobel Prizes but they never actually solved their problem 😃 ---
**Author:** ![D18](https://avatars.discourse-cdn.com/v4/letter/d/c57346/32.png) [@D18](https://boards.straightdope.com/u/D18)\ **Post date:** [April 25, 2012, 10:29pm UTC](https://boards.straightdope.com/t/where-is-the-cosmic-microwave-background/619928/6 "2012-04-25T22:29:36Z")
> [@Voyager](#): > > They might have gotten Nobel Prizes but they never actually solved their problem 😃 [They did indeed get their Nobels.](http://en.wikipedia.org/wiki/Discovery_of_cosmic_microwave_background_radiation) If I recall correctly, their paper was three or so paragraphs long. Is that some sort of record for the biggest prize for the smallest number of words? ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [April 26, 2012, 12:01am UTC](https://boards.straightdope.com/t/where-is-the-cosmic-microwave-background/619928/7 "2012-04-26T00:01:18Z")
As an aside, is anyone else reminded of the questions from the Baltimore Catechism? Q: Where is God? A: God is everywhere. ---
**Author:** ![Leo\_Bloom](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/leo_bloom/32/10377_2.png) [@Leo\_Bloom](https://boards.straightdope.com/u/Leo_Bloom)\ **Post date:** [April 26, 2012, 12:12am UTC](https://boards.straightdope.com/t/where-is-the-cosmic-microwave-background/619928/8 "2012-04-26T00:12:52Z")
A number of posters have mentioned the universe being essentially all light (photons), yet at the same time being opaque. I find this a stunning oxymoronic image. It is–precisely–a counterpart to Milton’s description of Hell in Paradise Lost: “darkness visible.” I presume the physicists/posters here are trying to express concepts like these as best as they can with analogies and metaphors. My question is: can you explore what an “opaque mass of photons” entails without reducing the metaphor–like what would I perceive if I could withstand the heat, radiation…–to the point of sillines? ---
**Author:** ![cmyk](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cmyk/32/3353_2.png) [@cmyk](https://boards.straightdope.com/u/cmyk)\ **Post date:** [April 26, 2012, 12:26am UTC](https://boards.straightdope.com/t/where-is-the-cosmic-microwave-background/619928/9 "2012-04-26T00:26:41Z")
I believe it really was, literally, a sea of heat and light. The universe was a luminous, roiling ball of matter and radiation. Like being _inside_ an explosion. ---
**Author:** ![Andy\_L](https://avatars.discourse-cdn.com/v4/letter/a/c67d28/32.png) [@Andy\_L](https://boards.straightdope.com/u/Andy_L)\ **Post date:** [April 26, 2012, 1:29am UTC](https://boards.straightdope.com/t/where-is-the-cosmic-microwave-background/619928/10 "2012-04-26T01:29:48Z")
> [@larryhughs](#): > > Is it everywhere, every point in every direction? How do we view it? I am very confused. According to NASA, some of the static that showed up on analog TVs was due to cosmic background radiation [http://www.nasa.gov/vision/universe/starsgalaxies/cobe\_background.html](http://www.nasa.gov/vision/universe/starsgalaxies/cobe_background.html) ---
**Author:** ![JWT\_Kottekoe](https://avatars.discourse-cdn.com/v4/letter/j/3ec8ea/32.png) [@JWT\_Kottekoe](https://boards.straightdope.com/u/JWT_Kottekoe)\ **Post date:** [April 26, 2012, 3:05am UTC](https://boards.straightdope.com/t/where-is-the-cosmic-microwave-background/619928/11 "2012-04-26T03:05:18Z")
> [@D18](#): > > [They did indeed get their Nobels.](http://en.wikipedia.org/wiki/Discovery_of_cosmic_microwave_background_radiation) If I recall correctly, their paper was three or so paragraphs long. Is that some sort of record for the biggest prize for the smallest number of words? The paper is a letter to the Astrophysical Journal and is about 10 paragraphs long, about a page and a half. It is directly preceded by a much longer letter by Dicke, Peebles, Roll, and Wilkinson that explains what Penzias and Wilson were seeing. That paper is entitled “Cosmic Blackbody Radiation.” The P&W paper is called “An Observation of Excess Antenna Temperature at 4080 Mc/s”. This is only one of R. H. Dicke’s several near misses at winning a Nobel prize. ---
**Author:** ![Leo\_Bloom](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/leo_bloom/32/10377_2.png) [@Leo\_Bloom](https://boards.straightdope.com/u/Leo_Bloom)\ **Post date:** [April 26, 2012, 3:09am UTC](https://boards.straightdope.com/t/where-is-the-cosmic-microwave-background/619928/12 "2012-04-26T03:09:29Z")
> [@cmyk](#): > > I believe it really was, literally, a sea of heat and light. The universe was a luminous, roiling ball of matter and radiation. Like being _inside_ an explosion. OK, let me take another stab at it. You have a photon measurement instrument; your eye, say. When you’re on the beach, there are photons on top of your arm, and fewer underneath, because you can see the shadow. You can see the sun against the darker, blue sky. The reflected light of the moon is brighter because of the darkness and pinpricks of stars. (I’m going somewhere with this.) Ditto sun from fairly closeup: shadow and background. Real close: shadow. **A** ) Now, is the suggestion–metaphor(?)–that 1) the nascent universe _as a totality_ is photons? **B** ) And, if 1) is correct, or close enough–whatever the hell that means–is the photon flux (or is it intensity that matters?) at the maximum that it physically can be? **C** ) And thus the observational methods I have listed (background, shadow) are useless, because there isn’t anything that \*isn’t \*photons, thus, “opaque”? ---
**Author:** ![Leo\_Bloom](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/leo_bloom/32/10377_2.png) [@Leo\_Bloom](https://boards.straightdope.com/u/Leo_Bloom)\ **Post date:** [April 26, 2012, 4:18am UTC](https://boards.straightdope.com/t/where-is-the-cosmic-microwave-background/619928/13 "2012-04-26T04:18:09Z")
Strike the above post. (Is there anything left, however, that seems interesting? Anything?☹ ) I’m chewing on this (typically poorly edited) W entry:[Cosmic time - Wikipedia](https://en.wikipedia.org/wiki/Cosmological_time#Recombination:_ca_377.2C000_years:) The electrons find their mates. I don’t understand what “are interacting” with means and why they were in the way of free photons. … As the universe cools down, the electrons get captured by the ions, forming electrically neutral atoms. This process is relatively fast (actually faster for the helium than for the hydrogen) and is known as recombination.[7] At the end of recombination, most of the protons in the universe are bound up in neutral atoms. Therefore, the photons can now travel freely (see Thomson scattering): the universe has become transparent. This cosmic event is usually referred to as decoupling. The photons present at the time of decoupling can now travel undisturbed (the photons’ mean free path becomes effectively infinite) and are the same photons that we see in the cosmic microwave background (CMB) radiation, after being greatly cooled by the expansion of the Universe. … **Before decoupling occurs most of the photons in the universe _are interacting_ with electrons and protons in the photon–baryon fluid. The universe is opaque or “foggy” as a result. There is light but not light we could observe through telescopes.** The baryonic matter in the universe consisted of ionized plasma, and it only became neutral when it gained free electrons during “recombination,” thereby releasing the photons creating the CMB. When the photons were released (or decoupled) the universe became transparent. The W entry “Photon Epoch” has much the same.