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Age of the universe and observable universe





yagnyavalkya
Is it possible to observe light that started about 14 billion years ago which means it is from the time of big bang
Can we detect the big bang
I just saw a TV series presented by Michio Kaku http://en.wikipedia.org/wiki/Michio_Kaku
in which he says there may be collisions between galaxies in fact he says that Milky Way and Andromeda might collide
he also says all the galaxies are receding that is the universe is expanding
It was also said that there is dark matter and it is detected by gravitational lensing of light
Do black holes also lense light
kelseymh
yagnyavalkya wrote:
Is it possible to observe light that started about 14 billion years ago which means it is from the time of big bang
Can we detect the big bang


Not exactly. The earliest "light" (radio waves) we can observe date from approximately 300,000 years after the Big Bang, when the universe had cooled enough for the hot plasma of protons and electrons to form neutral hydrogen atoms. This radiation is called the cosmic microwave background, and is the most definitive evidence we have that the Big Bang was real.

Quote:
I just saw a TV series presented by Michio Kaku http://en.wikipedia.org/wiki/Michio_Kaku in which he says there may be collisions between galaxies


Not "may." Astronomers have observed many, many galaxy collisions.

Quote:
he also says all the galaxies are receding that is the universe is expanding


Yes. This has been well known since the 1920's, when Edwin Hubble and others
noted the apparently linear relationship between the brightness of galaxies and the redshift of their light (spectral lines).

Quote:
It was also said that there is dark matter and it is detected by gravitational lensing of light.


That is one way to observe (or rather, infer) dark matter. It was originally hypothesized in order to explain why galaxies and clusters of galaxies are rotating much faster than is possible for them to stay together. If you just look at the visible matter (stars, dust and gas), then galaxies including our own should just fly apart from their rotation. But they don't (obviously)! Dark matter is what we call whatever the stuff is that adds extra mass to galaxies and clusters to hold them together.

Quote:
Do black holes also lense light


Yes. All matter lenses (or bends) light. That was a primary prediction of general relativity (1915), and it was observed in 1919 during a solar eclipse. Stars which should have been hidden just behind the edge of the Sun were visible because their light was bent around the Sun.
yagnyavalkya
Thanks
If both dark matter and black holes lense light how do we know which dark matter and which is a black hole
I mean if we detect dark matter by gravitational lensing of light how do we knwo it is dark matter for sure because black holes also lense light
kelseymh
yagnyavalkya wrote:
Thanks
If both dark matter and black holes lense light how do we know which dark matter and which is a black hole
I mean if we detect dark matter by gravitational lensing of light how do we knwo it is dark matter for sure because black holes also lense light


Do you know how a camera works? Do you know what "focal length" is, or "magnification"? Gravitational lensing is entirely similar -- from the pattern of images (and their distortions) we observes, we can calculate the distribution and density of matter causing them.
yagnyavalkya
kelseymh wrote:
yagnyavalkya wrote:
Thanks
If both dark matter and black holes lense light how do we know which dark matter and which is a black hole
I mean if we detect dark matter by gravitational lensing of light how do we knwo it is dark matter for sure because black holes also lense light


Do you know how a camera works? Do you know what "focal length" is, or "magnification"? Gravitational lensing is entirely similar -- from the pattern of images (and their distortions) we observes, we can calculate the distribution and density of matter causing them.

Thanks again
I may sound naive but if universe is receding then how do galaxies collide
kelseymh
yagnyavalkya wrote:
I may sound naive but if universe is receding then how do galaxies collide


That is a naive question, but not an unreasonable one. Another way to put it is this: "If the whole Universe is expanding, how does the Solar System stay together?"

The answer is quantitative: the scale factor (that is, the rate of recession as a function of distance from an observer) is 72 km/s per megaparsec. A megaparsec is 3.26 million light years (for comparison, our Milky Way galaxy is about 30 thousand light years across). 72 km/s is a very low velocity, all things considered. Escape velocity from the Earth is 7 km/s; escape velocity from the sun is 30 km/s.

The solar system, for example, is much tinier than a megaparsec: the orbit of Pluto is about 50 astronomical units (50 times the Earth's distance from the sun), or less than 250 trillionths of a megaparsec. In our local neighborhood, the cosmic expansion is about 17.3 microns/second! Gravity holds the solar system together. You can do similar arithmetic calculations for a galaxy (ten thousand or so parsecs), or even galaxy clusters (less than a megaparsec), and you'll see that gravity holds them all together.


Entities which are gravitationally bound together, such as planets, solar systems, galaxies, and clusters of galaxies, are not "pulled apart" by the cosmic expansion, because gravity holds them together locally. What is happening is that the various clusters of galaxies are receding from one another.
yagnyavalkya
kelseymh wrote:
yagnyavalkya wrote:
I may sound naive but if universe is receding then how do galaxies collide


That is a naive question, but not an unreasonable one. Another way to put it is this: "If the whole Universe is expanding, how does the Solar System stay together?"

The answer is quantitative: the scale factor (that is, the rate of recession as a function of distance from an observer) is 72 km/s per megaparsec. A megaparsec is 3.26 million light years (for comparison, our Milky Way galaxy is about 30 thousand light years across). 72 km/s is a very low velocity, all things considered. Escape velocity from the Earth is 7 km/s; escape velocity from the sun is 30 km/s.

The solar system, for example, is much tinier than a megaparsec: the orbit of Pluto is about 50 astronomical units (50 times the Earth's distance from the sun), or less than 250 trillionths of a megaparsec. In our local neighborhood, the cosmic expansion is about 17.3 microns/second! Gravity holds the solar system together. You can do similar arithmetic calculations for a galaxy (ten thousand or so parsecs), or even galaxy clusters (less than a megaparsec), and you'll see that gravity holds them all together.


Entities which are gravitationally bound together, such as planets, solar systems, galaxies, and clusters of galaxies, are not "pulled apart" by the cosmic expansion, because gravity holds them together locally. What is happening is that the various clusters of galaxies are receding from one another.

Thanks
yagnyavalkya
If it is that galaxies are not moving "through" space, but "with" space which means space is expanding then what about the space between nearby galaxies like Milky Way and Andromeda
They are on collision course
is the space between then expanding
metalfreek
yagnyavalkya wrote:
If it is that galaxies are not moving "through" space, but "with" space which means space is expanding then what about the space between nearby galaxies like Milky Way and Andromeda
They are on collision course
is the space between then expanding


The whole expanding universe is the expansion of space but not matter itself. You or earth or anything in universe will never get elongated or stretched. This expansion is of space. A balloon can be a good example of expansion.

The whole collision of galaxies are due to their gravitational interaction. Yes the space between milky way and Andromeda is expanding but their gravitational interaction will counter this expansion and take them to collision.

And by collision please be clear that only few actual collision will occur during combination because of the large distance between stars.
kelseymh
yagnyavalkya wrote:
If it is that galaxies are not moving "through" space, but "with" space which means space is expanding then what about the space between nearby galaxies like Milky Way and Andromeda. They are on collision course is the space between then expanding


Please re-read my next-to-last sentence,
kelseymh wrote:
Entities which are gravitationally bound together, such as planets, solar systems, galaxies, and clusters of galaxies, are not "pulled apart" by the cosmic expansion, because gravity holds them together locally.

and answer the questions below. You can find all of the answers using Wikipedia and a simple arithmetic calculator.

1) What is the estimated approach speed between the Milky Way and Andromeda, in km/s? For extra credit, derive your own estimate from the questions below.

1a) What is the heliocentric radial velocity of Andromeda, in km/s? This is the apparent speed between the Sun and Andromeda along our line of sight.

1b) What is the Sun's orbital speed around the center of the Galaxy, in km/s?

1c) Assuming the Sun is moving directly toward Andromeda, what would you estimate for the approach or recession speed between Andromeda and the Milky Way?

2) What is the current distance between the Milky Way and the Andromeda galaxy, in parsecs?

3) Using the nominal value for the Hubble constant I quoted, H0 = 72 km/s/Mpc, what is the "cosmic expansion" rate for the distance between the Milky Way and Andromeda?

Now, compare your answers to (1) and (3), re-read my earlier reply to you, and see if you can answer your own question.
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