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Comment by horsawlarway | Hacker News Reader

I agree with you here. A phrase that always echoes in my mind when discussing cosmology is

"All models are wrong - some models are useful"

The farther you venture from the verified useful section of a model (by which I mean - the farther you are from model predications that have been validated with observational evidence), the less you should trust it - ALL MODELS ARE WRONG!

And for most sciences - this isn't a huge deal - we can do lots of observational work easily right now. For cosmology... well - our observational data on the history of the universe it just astoundingly, mind-bogglingly, miniscule in comparison to the events we're interested in.

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Note that the most distant astronomical object we've observed has a redshift of 11, corresponding to a light travel distance of 13.4Gly. This means that if our models are correct and the universe is indeed about 13.8 billion years old, then we've seen 97% of the universe's history. Including the microwave background in that pushes the number to 99.997%.

....no?

We have a sampling of data from 99.997% of the timeline — but the sample itself is extremely sparse.

You have an observational slice of about 4000 years at the most generous, 400 years more leniently (telescope invention was ~1600s), 40 years for modern tools (OAO-2 in the late 1960s).

You might as well be looking at a single frame of a movie and telling me you know the whole plotline because it happens to have the same pixels on screen for the whole shebang.

But that specific frame has all the previous frames imprinted and increasingly faint in some sort of cosmological motion blur.

> the most distant astronomical object we've observed has a redshift of 11

And even though that seems like a large distance, it is a very small redshift compared to the total time the universe has existed. The cosmic microwave radiation background, for comparison, was emitted at a redshift of about 1100. So a redshift of 11 only covers 1 percent of the expansion of the universe since the CMBR was emitted.

> we've seen 97% of the universe's history

No, we haven't. See above.

> Including the microwave background in that pushes the number to 99.997%.

No, it doesn't, because even though we can see the CMBR, we can't see anything useful in between its redshift and the redshift of 11--not because there's nothing there, but because what's there is too distorted and faint to see. (The only reason we can see the CMBR is that it's black body radiation at a temperature we can independently predict from our knowledge of the physics of recombination, so we can tailor extremely sensitive instruments to looking for its precise signature.) So there is a lot of universe that we haven't seen.

No, we haven't. See above.

No we have. See above: While redshift goes all the way up to infinity, in terms of cosmological time, there's still only 0.4 billion years between z=11 and z=∞, or about 3% of the age of the universe.

To quote you above:

> if our models are correct

What is this whole discussion about?

You're arguing a stance that assumes the model is correct in a discussion about the risks of assuming the model is correct.

But trust me - our model is not correct.

> there's still only 0.4 billion years

In terms of time, yes. I was thinking in terms of observable spacetime volume. Even though there's only about 0.4 billion years before z = 11, the scale factor increased by a huge factor during that time (a factor of 100 from CMBR emission at z = 1100 to z = 11). That's a lot of spacetime volume from which we have no useful observations.

At what point does assumptions built upon assumptions approach being completely incorrect & useless?

The second you need to make a reliable predication based on it.

The good news (or bad depending on your take) is that we're nowhere close to being an advanced enough civilization to have any real stake in making predications about the future state of the universe other than "it will look basically the same as it does today", which we've conveniently used to position ourselves very accurately on the surface of the earth since the sextant and star charts.

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