Why hasn't the light from the CMB already passed us?

[Timvanhoomissen]

If the cosmic microwave background occurred at the moment that electrons and protons joined together and photons were now free to travel across the universe, then why haven't those photons already passed it. I'm having a hard time understanding why we can see the CMB forever if it was simply a moment in space time. For instance, if we look far enough back into space and time and observe the formation of a supernova, can't we only see that for a finite amount of time because there are only a certain amount of photons that emit the light of the supernova and travel to Earth? But with regards to the CMB photons in the microwave wavelength are constantly detectable, and I need some help understanding this concept.

 

[jbriggs444]

If the cosmic microwave background occurred at the moment that electrons and protons joined together and photons were now free to travel across the universe, then why haven't those photons already passed it. I'm having a hard time understanding why we can see the CMB forever if it was simply a moment in space time. For instance, if we look far enough back into space and time and observe the formation of a supernova, can't we only see that for a finite amount of time because there are only a certain amount of photons that emit the light of the supernova and travel to Earth? But with regards to the CMB photons in the microwave wavelength are constantly detectable, and I need some help understanding this concept.

A supernova occurs at a point. The CMBR was generated everywhere. As time passes we see CMBR that was generated in regions farther and farther away and which, accordingly, took longer and longer to get here from there.

 

[Timvanhoomissen]

A supernova occurs at a point. The CMBR was generated everywhere. As time passes we see CMBR that was generated in regions farther and farther away and which, accordingly, took longer and longer to get here from there.

But if there was a certain number of photons that were emitted from this event, why are we able to detect a steady stream of photons that lasts forever?

 

[PeterDonis]

if there was a certain number of photons that were emitted from this event

There weren't. The CMBR was generated everywhere, as @jbriggs444 said, and since our best current model says the universe is spatially infinite, that means "everywhere" is a space of infinite extent, so there were an infinite number of photons emitted.

 

[HelioGeo]

There weren't. The CMBR was generated everywhere, as @jbriggs444 said, and since our best current model says the universe is spatially infinite, that means "everywhere" is a space of infinite extent, so there were an infinite number of photons emitted.

Are you saying what goes around comes around?

 

[Chronos]

CMB photons we currently view were emitted when the universe was about 380,0000 years old and were emitted for a finite time interval in the infant universe, This creates the illusion that the last of those photons may someday pass us by. The universe according to GR, however, does not operate consistent with our customary notions of time and space. Due to expansion, CMB photons are hugely redshirted [z ~ 1090] so one seconds worth of CMB photons, according to our clocks represents a vastly shorter amount of time according to a hypothetically identical clock timing their emission at the surface of last scattering [SOLS] - IOW, we are watching CMB photon emissions in super slow motion. It took nearly 14 billion years of expansion for CMB photons to achieve their current redshift of z~1090. They will continue to increase in redshift as the universe continues to expand until they finally fade into oblivion in the distant future. In any event, keep in mind since the SOLS has long since receded beyond our cosmological horizon no CMB photons emitted after that time will ever reach us.

 

[PeterDonis]

Are you saying what goes around comes around?

No. In our best current model, any particular CMB photon only passes the Earth once.

 

[Grinkle]

@Timvanhoomissen This is a great question. Its a lay-person understandable reason that the observation of the CMB supports expansion, and supports that expansion happened everywhere. If it didn't, as you say, the CMB would (or will or might) disappear from view.

 

[Flatland]

But if there was a certain number of photons that were emitted from this event, why are we able to detect a steady stream of photons that lasts forever?

This was just explained to you. The light from the CMB occurred everywhere.

 

[Chronos]

While the big bang did indeed take place everywhere, the surface of last scattering [SOLS, the source of CMB photons] did not arise until later, when the universe was about 380,000 years old. The emission of CMB photons occurred over a finite period of time in the infant universe. We receive a steady stream of photons from that era because they were emitted when the SOLS was very near our cosmological event horizon. As an emitter recedes towards our cosmological event horizon its redshift approaches infinity and its photons become correspondingly time dilated. CMB photons are currently at a redshift of about z=1090, making the SOLS the most remote known photon source. We will continue receiving those photons forever. Think of it like Alice watching Bob fall into a black hole. She will never stop seeing Bob, nor will she ever see Bob reach the event horizon.