Dark Energy Question

[ThunderAeroI]

Perhaps this is a stupid question that proves I don't know anything, but let me at least try to ask it and see what comes up.

How can they differentiate between what is Dark Energy expanding the universe and the redshift of galaxies leaving the observable universe?

What I have seen is that they think Dark Energy really started expanding the universe over 5 billion years ago and it has increased in speed rapidly from there.

But I ask wouldn't the same visual clue exist as what is observable crossed into the non-observable sort of like the event horizon of a black hole.

I'd wager to say the thought is that since those stars are visible in a shorter distance than the CMBR they are not yet leaving the observable universe, but I'd like to know more.

 

[Asbo Zaprudder]

How can they differentiate between what is Dark Energy expanding the universe and the redshift of galaxies leaving the observable universe?

"They" are using both red shift and distance measurements to show that the recession started accelerating some billions of years ago. Red shift gives you a measure of the recessional velocity. It's the acceleration bit that led to the suggestion about some mysterious Dark Energy force being responsible.

What I have seen is that they think Dark Energy really started expanding the universe over 5 billion years ago and it has increased in speed rapidly from there.

Yep, but it's only a hypothesis.

But I ask wouldn't the same visual clue exist as what is observable crossed into the non-observable sort of like the event horizon of a black hole.

To an outside observer, nothing ever crosses the event horizon of a black hole. The rate of time elapsing on such an object appears to slow asymptotically and it gets more and more red-shifted. Gravitational fields also lead to a red shift due to clocks ticking at different rates for observers at different depths in the field (measurable, even in the Earth's field, by using the Mössbauer effect). From the object's POV, it passes through the event horizon (EH). Depending on the gravitational gradient, it might get ripped to shreds by the tidal forces before it even reaches the EH, of course.

I'd wager to say the thought is that since those stars are visible in a shorter distance than the CMBR they are not yet leaving the observable universe, but I'd like to know more.

Wouldn't we all...

Actually, very distant objects have already passed beyond the information horizon. Remember that we're now seeing them as they were a long time ago. The CMBR is a bit misnamed, as the radiation era existed everywhere in the Universe - more of a bath than a background.

Anyway, it's 30 years since I studied Cosmology so I could stand to be corrected.

 

[Asbo Zaprudder]

Sorry to bump the thread, but for those in the UK, Horizon on BBC2 is discussing these very issues tonight:

http://www.bbc.co.uk/programmes/b01mgllj