# How much larger will the "observable by us" universe be when JWST becomes operational?

Right now, using all our various current means of observing, we can "see" a sphere of X diameter around us. Webb will increase that to Y diameter. So our observable volume will increase by some percent (Volume Y - Volume X) / Volume X.

I'm not sure I can rely on any values I find for X and Y using google searches. As of April 2022 what is the diameter of what we can presently observe and what is the expectation for JWST?

ADDITION: my apologies for not having enough subject matter expertise to frame my question as well as it might have been. I was thinking along the lines of pela's response. With JWST having a better set of instruments etc our practical ability would have farther reach. I've known there were physics limits to what could ever be observed that could not be changed. the CMB is not the type of observation I had in mind when asking the question.

• On what grounds do you presume that the observable universe should get bigger? Travel time and speed for light is the same, irrespective of wave length. Apr 2, 2022 at 22:23
• I think the OP is referring to not the the theoretically observable Universe, but the "practically observable Universe of objects (i.e. not including the CMB)".
– pela
Apr 2, 2022 at 22:44
• Considering OP explains that he is a layperson, I find it pretty easy to interpret this question as "how much larger will the sphere/volume of practically observable space in the wave length visible to JWST be" and to explain this interpretation in the answers... I find the question perfectly valid and understandable from the POV of someone who does not have much knowledge about these things. @JamesK
– AnoE
Apr 4, 2022 at 11:37

tl;dr Conservatively 10%, realistically 25%, optimistically 60%.

I assume that by the "observable-by-us Universe", you mean not the theoretically observable Universe, which is given by the distance light has had the time to travel since the Big Bang, but the part of the Universe where we may practically see galaxies and other objects, not including the cosmic microwave background.

### Current redshift record

The currently most distant, spectroscopically confirmed object is the galaxy GN-z11 (Oesch et al. 2016), which has a redshift of $$z=11.1$$ and hence a distance of just over $$d=32$$ billion lightyears (Glyr). The volume inside the sphere with us in the center and GN-z11 on the surface is $$V = 4\pi d^3/3 \simeq 140\,000\,\mathrm{Glyr}^3$$.

### How far will James Webb see?

Predicting the redshift record for James Webb is not easy, since it depends on the unknown physical conditions of even younger galaxies. The important quantity is their (surface) brightness which depends not only on intrinsic properties such as their star formation rate, dustiness, and compactness, but also on the properties of the intergalactic medium, in particular to which degree it is (re-)ionized and hence able to transmit the light from the galaxies. Moreover, we also don't know how many there, so estimates rely on extrapolating their distributions of luminosities (luminosity functions) from lower redshifts.

Rather conservative predictions for Webb's redshift record are around $$z\simeq13\text{–}16$$ (Mashian et al. 2016; Williams et al. 2018; Mahler et al. 2019; Behroozi et al. 2020), corresponding to an age of the Universe of 250–300 million years after the Big Bang (whereas GN-z11 is seen ~400 Myr after the BB), and a distance of 33–34.5 Glyr.

These estimates hence correspond to volumes of $$\simeq 150\,000\text{–}170\,000\,\mathrm{Glyr}^3$$, i.e. an increase of up to roughly 10–25%.

However, sometimes we're lucky and line up with a massive cluster of galaxies acting as a gravitational lens which magnifies the light from distant background sources by factors of tens, hundreds, or even thousands. This may potentially allow James Webb to see some of the very first galaxies, predicted to have formed at $$z\sim20\text{–}30$$, 100–200 Myr after BB. I don't know of any peer-reviewed papers doing any serious estimates, but NASA routinely cites Webb's predicted record as "200 (or 250) Myr, possibly even 100 Myr" af the BB (e.g. here). If we really discover a galaxy 100 Myr after the BB, that would correspond to $$z=30$$, and a distance of 38 Glyr, i.e. a volume of $$225\,000\,\mathrm{Glyr}^3$$ which is 60% larger than our current "observable-by-us Universe".

The extension of our probed volume may not seem like a lot. A better way to understand the significance of this is to think about the age of the Universe at that time: It is likely that we will make the unknown epoch of the Universe half, or maybe even a quarter, of today's value.

For comparison, the volume inside the region from which the cosmic microwave background was emitted at $$z=1100$$ is $$390\,000\,\mathrm{Glyr}^3$$, while the volume of the (theoretically) observable Universe is $$413\,000\,\mathrm{Glyr}^3$$.

The figure below is pretty much to scale: • "The figure below is pretty much to scale:" - apart from the size and density of galaxies which is vastly out of scale ;-) Apr 3, 2022 at 11:30
• @asdfex AND... does not show home of LGM Apr 3, 2022 at 19:56
• @asdfex Okay, the galaxies aren't, but the telescopes are indeed 4 billion lightyears long ;-)
– pela
Apr 3, 2022 at 22:09
• Dr Becky just explained the gravitational lens technique you mentioned, which Hubble used to spot the most distant star ever seen.
– Wyck
Apr 4, 2022 at 13:46
• Could you expand your answer to estimate approximately how the improvement in angular resolution and ability to pick up weak signals will affect how distant galaxies the telescope will be able to pick up? Supposedly there is no longer a linear relationship between redshift and distance/time so no matter how redshifted objects you pick up there needs to be no change in the estimate of time passed by since the big bang etc? Apr 27, 2022 at 9:54

Not by very much. You can use an online calculator to convert redshift to distance. Assuming that a redshift of z=11.1 was the most redshifted object found before James Webb, this corresponds to a light travel time of 13.306 billion light years. If Webb wore to find an object at z=25, which is probably beyond what it is able to achieve, this would correspond to a light travel time distance of 13.59 billion light years.

You can then easily calculate (13.59/13.306)^3 and end up with an increase in observed volume of about about 6,5 percent.

Now if James Webb wore to find a galaxy with an infinite redshift, which it obviously will not, this would correspont to a light travel distance of 13.72 billion years. You can do the math (13.72/13.306)^3) and end up with an increase in volume of about 9.6 percent.

Answer: Even if James Webb was to find an object with a redshift of z=25, which is more than double the pre James Webb value, the observable volume will just increase with about 6.5 percent. This is due to the fact that using the current model of converting redshift to distance at high redshifts the distance to the point of emission of the observed light will vary very slowly with increase in redshift.

• Sorry, this answer is not correct. You're calculating a "volume of time", which would only make sense if the Universe didn't expand, in which case it would be equivalent to the real volume.
– pela
Sep 28, 2022 at 20:40