# Size of universe

## Main Question or Discussion Point

How do we know that observable matter is spread over a space at least 93 billion light years across the universe. How one can measure such distances? We can only observe light which
was emitted long time ago so how we can predict current distance of such object.

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diazona
Homework Helper
I think you're confusing the size of the universe with the distance to the sun (about 93 million miles). The most distant object ever seen was something like 8 billion light years away, last I checked, though I have a feeling that record might be a little higher now. That figure of 8 billion light years comes from various sorts of measurements, like the Doppler shift and Hubble's law (that on average, the further away something is, the faster it's moving).

Of course, that distance does mean that the light has traveled 8 billion light years through space, over a time of 8 billion years - it doesn't mean that the object is 8 billion light years away right now. Of course it will have kept moving further away after it emitted the light.

Chronos
Gold Member
The most ancient photons in the observable universe [CMB] were emitted by the surface of last scattering about 13.7 billion years ago. The distance 'now' to that surface is model dependent. It is believed to be around 150 billion light years based on LCDM parameters.

337
Hi Chronos, is there any way to determine the age of a photon (from a *stationary reference point) ?

IMO determining the present size of the universe depends on how we define "present", the present we see today is only from our local reference point, so the further away we look, the further "back in time" we look, but if we traveled to any other point in the universe to perform the observation - the exact same thing could be said.... sylas
How do we know that observable matter is spread over a space at least 93 billion light years across the universe. How one can measure such distances? We can only observe light which was emitted long time ago so how we can predict current distance of such object.
Your value of about 93 billion lights years is correct... it is what you get using the current default values in the ΛCDM model used in Ned Wright's much loved http://www.astro.ucla.edu/~wright/CosmoCalc.html" [Broken]: Ωm = 0.27, ΩΛ = 0.73, H0 = 71 km/s/Mparsec. This is twice the current proper distance to the our "Hubble volume", the space within which is now the "observable universe", all the matter we can see.

You get this radial distance by using a large value of z (a million, say) and looking at the "comoving radial distance" in the calculator results. z of 1100 corresponds to the current distance to the surface of last scattering, which is the source of the cosmic background radiation.

Predicting such distances simply comes from solving the equations for scale of the universe using the conventional Big Bang models... the FRW equations. This is how the calculator does it.

Cheers -- sylas

Postscript for a reply to 337:
IMO determining the present size of the universe depends on how we define "present", the present we see today is only from our local reference point, so the further away we look, the further "back in time" we look, but if we traveled to any other point in the universe to perform the observation - the exact same thing could be said.... The universe is "now" about 13.7 billion years old. The distance to another point in space "now" is a distance to another point in space/time which also concludes that the universe is 13.7 billion years old, same as we do. Of course, what we see is from the past. If we see a galaxy with light that left a galaxy when the universe was only 3.34 billion years old, the cosmological redshift would be z=2, and the distance to that galaxy NOW would be 17.1 billion light years.

I'm using "proper distance" co-ordinates, and Ned's calculator with its default ΛCDM parameters as given above.

Cheers -- sylas

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337
Thanks for your response Sylas, however, this model assumes that the object maintains a constant-velocity motion (no forces act on it) and that the rate of time passage is constant.

In an expending universe with a fixed amount of matter / energy, but an increasing volume, I question the validity of these assumption because the object must feel some forces (gravity ?), and concentrations of matter / energy may be changing as well in the region of the object, in our region, anywhere in between (relativistic effects).

It seems very complex....

sylas
Thanks for your response Sylas, however, this model assumes that the object maintains a constant-velocity motion (no forces act on it) and that the rate of time passage is constant.

In an expending universe with a fixed amount of matter / energy, but an increasing volume, I question the validity of these assumption because the object must feel some forces (gravity ?), and concentrations of matter / energy may be changing as well in the region of the object, in our region, anywhere in between (relativistic effects).

It seems very complex....
It is complex; but not as complex as you might think.... once you get used to some initially counter intuitive notions.

In fact, this model does assume forces at work. Gravity pulls things together, and "dark energy" pushes them apart. As the scale factor of the universe increases (roughly, as the universe expands) the density of matter drops, and this reduces the impact of gravity. But "dark energy", also called "cosmological constant", remains the same. This is why cosmologists speak of "accelerating expansion". Once the universe has expanded enough that the impact of dark energy is strong enough by comparison to the impact of gravity, then the rate of expansion starts to accelerate. The universe is, it seems, now in this state of accelerating expansion. But earlier, when the universe was more dense, the rate of expansion was slowing down.

The calculator I mentioned basically solve the equations for rate of expansion, as governed by these forces, and this in turns lets you infer such things as how old the universe is, and how far it is to objects we see at extreme distances, given their redshift.

To get to grips with this, you start slow and work your way up. There are various resources that can help; one of my favourites is Ned Wright's Cosmology Tutorial, which is in the same pages as the calculator.

Cheers -- sylas