If all goes well, JWST will allow us to witness events that occurred roughly 180 million years cosmic time (after big bang). Apparently this light arrives 13.6 billion years after it was emitted.

In my naivety I would expect the universe at that time to be roughly 180 million light years in radius, and the the light to take 180 million years to reach us. Apparently it takes 75x longer.

My guess is that we can attribute this to the universe more rapidly expanding in its beginning. Or maybe it's because space is stretching while the light is traveling towards us. Or is it because space and time are deformed according to laws or relativity?

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    $\begingroup$ Simple explanation. Space grows. Nowadays, it grows at a rate much faster than lightspeed. Universe is only ~14 billion years old, but is 93 billion light-years wide. $\endgroup$ Jan 9 at 13:53

2 Answers 2


Your second guess is the right one. Light started to travel when the distance was only 180 Mly (or something in this ballpark, due to a not fully constant expansion speed). But while the light traveled, the space between us and the source expanded, so the total distance traveled increased to 13.8 Gly.

Deacon Vorbis made this nice animation for Wikipedia (intended for an related mathematical problem, the "Ant on a rubber rope", but it still fits) Ant traveling along an expanding rubber rope https://commons.wikimedia.org/wiki/File:Ant_on_a_rubber_rope_animation.gif

Please let me point out one peculiarity: In the animation the distance between us and the source increases with twice the speed of light! And still light is able to reach us, even though it travels slower than the distance increases. This also implies: Light needed 13.8 Gly to travel, but the actual distance to the source today is a lot larger now.


The 180 million years have nothing to do with the age and size of the universe.

The Big Bang was 13.6 billion years ago. The radius of the observable universe is about 46.5 billion light-years.

Due to the red shift the JWST may observe light emitted 180 million years cosmic time after the Big Bang. The red shift factor is about z = 20, that means the wave length of the emitted light was 20 times shorter than that infra red radiation received by the JWST. The JWST receives wavelengths from 0.6 to 28.3 μm.

So the 180 million years are the result of the red shift and the wavelength range measured by the JWST.

The wave length of light emitted less than 180 million years after the big bang is now after the red shift too long for the JWST. This radiation is in the microwave range and not in the infra red range.


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