Science a GoGo's Home Page Forums General Discussion Physics Forum Expansion of the Universe

What is the nearest object (e.g. galaxy) from which a red shift calculation can be made which realistically demonstrates that the Universe is expanding?

Redshift is observed in the spectra of virtually all galaxies 30m lt yrs or more away. The realistic demonstration that the implied recessional velocity is due to the expansion of space is: the dimmer the light from 'standard candles', the greater the redshift. Owing to the effects of gravitation, the redshift of a single more local object, though it might be consistent with the expansion rate, may or may not be entirely attributable to the expansion.

Thanks Rede. Does this mean that the best we can say is that 30m years ago, the Universe was expanding, but we don't actually know what it's doing now?

It may not be the very best that can be said, but it appears to be true. In astronomy we're reading yesterday's news. As with most things, we use the word 'is' but strictly speaking we're expressing a judgement based on available data. In science, that amounts to a rational belief system subject to change.

I appreciate that 30m years is insignificant in terms of the age of the Universe. It’s a bit like saying “the Universe was expanding yesterday, but it could have stopped last night”.

I guess, if the expansion was accelerating as recently as that, it is unlikely to have made any major change yet.

That's right. Radio frequency observations, which have allowed us see into the past to about 380,000 yrs after the Big Bang, indicate that space has always been expanding; and that's backed up by very accurate measurements of the CMBR that fit the predicted value (in every direction). It would be astounding if it had stopped, for some inconceivable reason, at about the moment we arrived on the scene.

We might have to take our share of responsibility for climate change, but I agree stopping the expansion of the Universe could be a bit beyond us.

On the other hand, there is this theory.......don't go there!

I read in some article recently that the universe had stoped expanding

Hi, uros. There may or may not be scientific data to support the notion that the universe has stopped expanding, but one needs to note the source of the information and weigh its credibility accordingly. As you've probably noticed, the internet is flooded with nonsense. It might be possible to find nonsense to fit any conceivable crazy idea.

If we were able to detect signs that the Univere had stopped expanding, how long ago would the expansion have had to stop?

I'll take a stab at it, Bill. Since the nearest galaxies observed to be receding in accordance with the Hubble Constant** - and virtually unaffected by local intergalactic gravitational attraction - are about 30m lt yrs, away, I think we can take the answer to be about 30m yrs ago.

As we suggested previously, it's very improbable that after 13.7 billon yrs an end to expansion would happen at the very moment we became capable of detecting it - even given some feasible hypothesis regarding how it might occur.

** "With some slight modifications to the Cepheid scale zeropoint, we believe our best value for the local H0 determination is around 71 (+/- 7) km/s/Mpc"

https://www.cfa.harvard.edu/~dfabricant/huchra/hubble

If the Universe had stopped expanding about 30 m y ago, it seems unlikely that it would have remained static. Whatever force stopped the expansion is going to have reversed the movement. If the Universe had been shrinking for the past 30 m y, isn't it likely that we would be aware of the approach of closer objects, even if we had not been hit by something?

Well, it would take quite a while for the contraction to have started showing any great signs. Rede found a value for the expansion of the universe of 71 k/s/megaparsec. 1 kiloParsec = 3,261.6 light years, so 1 mPs is approximately 3.3 million light years. The recession speed at that distance would then be 71 kps. Considering the distances involved that is a real crawl. So there wouldn't be any obvious signs at this time. There definitely wouldn't be anything running into us yet.

Bill Gill

Absolutely right. I had overlooked the fact that it is the galaxy groups that are in relative motion, so the distances would be vast.

Yes, the scales of intergalactic time and space are vast, even within the local group. Regarding which, and on the subject of things actually hitting us, this is interesting:

"The Andromeda–Milky Way collision is a predicted galaxy collision that could possibly take place in approximately 3 to 5 billion years' time"

"Nevertheless, the chance of even two stars colliding is negligible because of the huge distances between them"

"Two scientists with the Harvard–Smithsonian Center for Astrophysics stated that when, and even whether, the two galaxies collide will depend on Andromeda's transverse velocity. Based on current calculations they predict a 50% chance that in a merged galaxy the solar system will be swept out three times farther from the galactic core than it is currently located. They also predict a 12% chance that the Solar System will be ejected from the new galaxy some time during the collision. Such an event would have no adverse effect on the system and chances of any sort of disturbance to the Sun or planets themselves may be remote.

Without intervention, by the time that the two galaxies collide, the surface of the Earth will have already become far too hot for liquid water to exist, ending all terrestrial life, which is currently estimated to occur in about 1.4 billion years due to gradually increasing luminosity of the Sun."

http://en.wikipedia.org/wiki/Andromeda-Milky_Way_collision
http://www.scientificamerican.com/articl...f-far-from-home

Rede, you say You mean the cosmos turned to communism? .
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LIFE, GAMES, BASEBALL and BIG-innings--just a short story. As a lover of astronomy and the games of life, I hope it is not an intrusion.

BTW, anyone, if you are a sports-fan you are no doubt aware that Christopher Moses, is the new sports-writer of the Globe & Genesis Daily News. The headline of the lead-story by him in today's G&G SPORTS is:
THE LONG WAIT IS OVER.

"Finally", CM said, "someone--and that someone is an amazing newcomer--about whom I will say more, later--has brought the buzz back to what has been a dismally-dark season for the cellar-dwelling Earthlings.

"The Earthlings accomplished this win against the heavy-hitting Martians, despite the fact that the Martians had a two-run lead up to the seventh inning of a crucial game in the playoffs, which the ... went on to win. And let's not forget the superb pitching by the tall guy from Porto Rico, Jesus Diabolos--the best pitcher of the Martians.

J.C. MAKES GOOD
"How did this happen? Back to the new comer. In a word: Jay C. Good. JCG, though he was new to the team and a rookie to the game, he soon learned how to play it, well. Furthermore, a team player, he was respected by all his fellow players and loved by his fans.

"In addition, he was active, not just in his sport, but in the giving of himself in service to the whole community, especially the needy in Porto Rico. Of course there were those who were jealous of his talent. Also, not everyone was happy when he spoke up for the under-dog and the need for justice for all, including helping injured athletes who were not as fortunate as was he. Back to the game.

"In the seventh inning, with two out and bases loaded, JCG was given his first opportunity at bat.

"The call was: Strike!..Ball!..Ball! Strike! Ball! ... then WHAMMM!!! With this grand-slam home run, out of the cellar the Earthlings began to come. And the rest is history.
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Later, in a book on baseball, Chris Moses gave special mention to Jay C. Good and the kind of life he lived. He titled the first chapter: "In the BIG-inning ..."

No wonder that, from then on, the fans of the Earthlings shouted in unison at every game: LET THERE BE LIGHT!!! With the new buzz JCG brought to the game, it was light-like expansion from then on, for all involved.
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Well, scientists sometimes remind us that science isn't democratic, so maybe there's a cosmic principle in there somewhere