# Question about the speed of light, photons and redshifting?

The universe was created 13.7 billion years ago, the radius of the observable universe is 46 billion light years.

Can this apparent anomaly be explained by redshifting, as is sometimes claimed to be the case?

If so, when is a photon redshifted? If all photons in a beam of light are redshifted then C wouldn’t equal C at all, unless C is a value that includes redshifting (does it?), in which case, the explanation of photons travelling 46 billion light years in 13.7 billion years isn’t valid.

Can anyone assist with this and clarify the time/distance anomaly and put the role of redshifting into context?

Relevance

The anomaly isn't explained by redshift - rather, it is explained by the metric expansion of space, which causes redshift.

Basically, the photons that were emitted 13.7 billion years ago are just reaching us - but the objects that emitted those photons are now 46 billion light years away, since space has been created in between us and them.

Imagine you are on a train track travelling from point A to point B at some fixed speed. In the middle of your journey, someone constantly adds more track in between you and your destination. When you eventually reached your destination, you measure the track length again - and you noticed that it's much longer than it started. That's how you can have the discrepancy between the speed and the actual distance.

Redshifting does not, in anyways, change the speed of light. In fact, it is the result of keeping the speed of light constant - and phenomenon observed to be correct.

The other poster, Ahmet, should note that the figure of 46 is correct for the radius of the observable universe, which is what's pertinent to the discussion at hand. We're discussing photons from the edge of the observable universe just reaching us, the centre of the observable universe. The other figure he's trying to get is 93 billion light years, the diameter of the observable universe.

• Ah, the red shift is not your classic Doppler red shift. The red shift due to universal expansion is due to a lengthening of the metric we measure distance (e.g., wavelength) by.

Without the expansion, the radius of the observable universe would be just that 13.7 billion light years (minus a few years when photons were not present right after the big bang). That would be using the local distance metric (e.g., the light year in meters).

But as the universe is expanding, and going faster each day, the metric in the older parts of the universe that have been expanding longer than our little piece of it, is longer than our metric. So, for example, a wavelength of l = 1 meter here on Earth, might be measured as L = 3 m using our measuring stick.

And, as light speed, C, is C no matter where or when, we have wavelengths l = C dt and L = C dT, where dt and dT are corresponding periods, so that F = 1/dt is the frequency for l here on Earth, while f = 1/dT < 1/dt = F is the lower, red shifted, frequency for the lengthened metric of expanded space.

BTW that 46 billion light years reflects the longer metric of old expanded space, but as measured in our current metric here on Earth with comparatively new space.

• The 46 billion light year figure is actually 96 billion light years.

The "anomaly" is due to hubble's law, which states that space-time is relative to the exponential constant which is the expansion of the universe.

While the common sense view is that if the universe is 13.7 billion years old, then we should only be able to see 13.7 billion light years this is wrong, because this logic would only work in a flat universe where expansion is uniform and is the same at one point as it is at another.

• I think you are confusing the concept of the Doppler shift with a reduction in the speed of light. The red shift refers to the shift in the frequency, or the color of the light as it appears to us. The speed of the light is always the same, but it's color seems to shift to the red end of the spectrum because the source of the light is moving away from us, or we are moving away from it, or both. The light seems to be "stretched out", the frequency seems lower and so the light looks redder.

As for the size of the "Observable Universe", this may be different than the actual size if you calculate 13.7 billion years times the speed of light per year. Our ability to observe is limited by our level of observing technology, so to speak. This is a very simplistic answer, I know, but if I tried to get more technical, I would probably end up tripping all over myself. Hope this helps.

• Red-shift happens BECAUSE the speed remains constant and space is expanding.

The 'red' is a frequency NOT a speed.

Expanding space increased the space between 'peaks' of that wave

as it traveled at that set speed. - The frequency changed. The speed didn't.

Note that 'C' is a LOCAL limit, defined only in the space the photon occupies.

That expansion means that we can 'see' objects receding in different directions,

whose combined velocities of recession exceed 'C'.