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Comment by readthenotes1 | Hacker News Reader

"This extrapolation predicts that the cosmos should currently be expanding at a rate of 67.4 km/s/Mpc, with an uncertainty that’s less than 1%."

I can't measure my own weight with an uncertainty that's less than 1%. I wonder what these peeps are on...

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Depending on which end of the scale you are interested in, the NIST would be an interesting place to work.

How To Measure The Tiniest Forces In The Universe https://youtu.be/pXoZQsZP2PY and World's Heaviest Weight https://youtu.be/_k9egfWvb7Y - both from Veritasium.

From the expanded description on the heaviest weight:

> Before visiting NIST in Washington DC I had no idea machines like this existed. Surely there's an accurate way to measure forces without creating such a huge known force?! Nope. This appears to be the best way, with a stack of 20 x 50,000 lb masses creating a maximum force of 4.45 MN or 1,000,000 pounds of force. I also wouldn't have thought about all the corrections that need applying - for example buoyancy subtracts about 125 pounds from the weight of the stack. Plus the local gravitational field strength must be taken into account. And, the gravitational field varies below grade. All of this must be taken into account in order to limit uncertainty to just five parts per million (.0005%)

Skill issue.

What is "Mpc"., if anybody knows?

megaparsec (1 million parsecs)

Thanks much

It is a distance unit, megaparsecs.

Or approximately 3.25 million light years.

Appreciated. Big distance

I have difficulty conceptualizing "distance over time ... over [huge] distance"

I guess it means "chunks about so [megaparsecs] large are moving [themselves] at a speed of so many Km. per second" but I could be wrong

The measurement for expansion is linear with distance, so two spots one Mpc from another moves away front each other at 67.4 km/s while two spots two Mpcs from each other moves at 134.8km/s. This means the expansion is accelerating and some parts of the now visible universe will eventually move away from us faster than the speed of light resulting in them disappearing from our view.

The distances, time and speeds are indeed very hard to comprehend from our usual references :)

It's crazy that some parts of the universe will actually for all purposes vanish

Thanks for taking time to break it all down

At some point there will only be the galaxies in our local group visible, it's interesting to imagine a future civilization only having such a limited universe to view.

Doesn't this mean that on a long enough time frame, an observer anywhere in the universe won't be able to see anything because everything else in the universe is too far away to be visible?

Simply - yes. Furthermore, civilizations that arise in that era of the universe will likely have a different cosmology than what we are able to understand today. If you could only see the galaxy that you are in, you wouldn't be able to see galaxies that were forming shortly after the Big Bang, or be able to use supernovas in other galaxies to measure the scale of the universe.

Kurzgesagt did a video on that - TRUE Limits Of Humanity – The Final Border We Will Never Cross https://youtu.be/uzkD5SeuwzM

>> civilizations that arise in that era of the universe will likely have a different cosmology than what we are able to understand today.

That is just mind-boggling

It means that a chunk of space with length 1 megaparsec will be 67.4 km longer a second later. If you divide that new length by the old length, you get the factor by which space expands each second. It’s a very small factor (i.e. very close to 1), but there are also many seconds.

Reading your very confident response it makes me wonder the same about you.

Their wallets are much bigger than yours.

As is what they are trying to measure. I don't believe 1% measurement error in any universal element except perhaps the speed of light...

That's an absurd statement. For example, planck's constant is known to better than 1%, as is the mass of various particles. Heck, the Earth, which is sufficiently non-spherical for it to matter only differs in radius (between polar and equatorial) by 0.3%!

I suggest you take a look at this list of physical constants, paying special attention to the "uncertainty" column, and then get back to us on why you don't accept any of them except the speed of light.

https://en.wikipedia.org/wiki/List_of_physical_constants

Here's a nice list: https://en.wikipedia.org/wiki/List_of_physical_constants

G (the gravitational constant) is an interesting one: the value is only known to about 5 significant figures, but GM (the gravitational constant multiplied by the mass of the Earth) is known a lot more accurately, unsurprisingly, considering how well GPS works. Some of those constants seem to be known to about 12 significant figures.

If you can measure the speed of light extremely precisely, you can measure a lot of constants extremely precisely.

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