Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Novaflipps on 29/11/2016 21:17:07
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Could we be in the process of the big crunch as we speak?
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No at the present time the Universe is still expanding and until recently it was thought that the rate of expansion was increasing but the most recent measurements throw doubt on this but as yet there is no evidence that it is slowing down.
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But would not the observation of the universe look the same either way, the redlight displacement. Since if we move away from everything and everything moves away from us. And the acceleration would be faster the closer to the center of the big crunch?
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But would not the observation of the universe look the same either way, the redlight displacement. Since if we move away from everything and everything moves away from us.
It is true that redshifted light shows that most of the universe is moving away from us.
This means that the universe is still expanding away from a Big Bang.
If we were moving towards a Big Crunch, most of the universe would be moving towards us, and most light would be blue-shifted.
So recent Big Bang and impending Big Crunch look quite different.
And the acceleration would be faster the closer to the center of the big crunch?
One of the more difficult concepts of the Big Bang is that the whole universe exploded, so everything is moving away from everything else, so there is no obvious "center".
In the universe today, some local effects like collapse of a dust cloud in space can lead to a local (little) "crunch" that produces stars and (eventually) black holes.
But it is thought that a Big Crunch would involve the whole universe, and we would see more distant parts of the universe approaching us more rapidly than closer parts, with no obvious "center".
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Of course, we could be experiencing reversed time in a shrinking univers. :)
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I understand the concept of the big bang and everything moves away from us. But since the universe is so unimaginable big, and that there is no defined center, hmm. Let's throw three marbles of light towards a black hole. We are the middle one, and we look back at the marble further away from the black hole. We move away from it cause of the acceleration towards the black hole and get the red shift effect. Then we look at the marble closest to the bh. It moves away from us cause the gravity is even greater. And we get the redshift effect. All 3 marbles goes towards the black hole, to the center, but still moving away from each other. Ofc some point they all will merge, but that's not my point
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The observable universe extends to some 13.7 billion light years radius. We observe red shift in the most distant objects. This means that they were moving away from us 13,700,000,000 years ago. We have no idea what they are doing now.
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The observable universe extends to some 13.7 billion light years radius. We observe red shift in the most distant objects. This means that they were moving away from us 13,700,000,000 years ago. We have no idea what they are doing now.
If there is a star and an observer in a spaceship one light year away. There is no relative motion between them.
The light from the star is red shifted due to the star gravity.
If the observer is moving very slowly towards the star so the Doppler blue shift is a fraction of the gravitational red shift then the star light would still be red shifted.
Is there a way to recognize these two red/blue shift effects and to determine the proper relative velocity?
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Is there a way to recognize these two red/blue shift effects and to determine the proper relative velocity?
Yes.
- Gravitational red shift only becomes significant if you are looking at high-mass, compact objects like white-dwarf star, a neutron star, or accretion disk around a black hole (the black hole itself being effectively invisible).
- "Normal" stars have negligible gravitational red shift, so you can interpret any red or blue shift as a relative velocity
- Normal stars and compact objects look pretty distinctive, so you are unlikely to confuse them
- The spectrum also gives away information like temperature, chemical composition and magnetic field, which also helps to diagnose what type of object is being examined.
- The high-gravitation stars are also physically small (small surface area emitting radiation), so they are almost invisible at long distances - unless you can spot polar jets from an accretion disk or blips from a pulsar, using radio astronomy
But for high-resolution spectroscopy applications like discovering extrasolar planets, the absolute redshift does not matter - what matters is the change in redshift when measured over days and decades (after you subtract the velocity redshift due to Earth's rotation, and the orbits of Earth, Jupiter and Saturn).
The first measurement of gravitational redshift was made on white dwarf Sirius B, in 1925.
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I understand the concept of the big bang and everything moves away from us. But since the universe is so unimaginable big, and that there is no defined center, hmm. Let's throw three marbles of light towards a black hole. We are the middle one, and we look back at the marble further away from the black hole. We move away from it cause of the acceleration towards the black hole and get the red shift effect. Then we look at the marble closest to the bh. It moves away from us cause the gravity is even greater. And we get the redshift effect. All 3 marbles goes towards the black hole, to the center, but still moving away from each other. Ofc some point they all will merge, but that's not my point
The problem is that you are only considering the line of sight that goes directly in towards or away from the BH. What of the marble that starts at a point an equal distance from the Black hole, but some distance from you along the circle defined by that radius. As you both fall in, you maitain the same distance from the BH, but the circumference of the circle you share is getting smaller. You would see that marble as moving towards you and would measure a blue shift from you. Your model of objects falling in towards a common center would not produce a red-shift in all directions like we observe now.