| Topic: |
Science > Physics |
| User: |
"postman" |
| Date: |
18 May 2007 03:37:57 PM |
| Object: |
My lack of understanding of the Big Bang Theory |
As a lay person I find it very difficult to understand the big bang theory.
1: if you point the Hubble space telescope at a point in space and leave
it for long enough it will eventually gather enough light to show a
galaxy that is perhaps 12 billion light years away.
If you then swing the telescope 180% in almost anyway plane and repeat
the procedure you will again see a galaxy 12 billion light years away.
These two galaxies are apparently 24 billion light years apart but 12
billion years ago the universe wasn't 24 billion light years across
according to the BBT.
2: Cosmic Background Radiation .. My limited understanding of radiation
is that it starts with an event, consists of photons, travels in
straight lines and travels at the speed of light. It does not sit in the
background. The event happened approx 14 billion years ago and unless we
have been travelling a lot faster than the speed of light for the past
14 billion years we should not be experiencing this radiation. Even if
we have travelled faster than the speed of light due to expansion then
the radiation would also have been speeded up due to that expansion. If
my admittedly limited understanding is correct the event that created
the CMBR actually happened quite some time before the galaxies were
formed. So the CMBR would have passed the position the galaxies were to
form in before they were even born. If the CMBR bounced of the edge of
the universe then does all radiation bounce of the edge of the universe
and if so should we be seeing mirror images of galaxies whose light has
been bounced in a similar way. If it does bounce how many times will it
have bounced in 14 billion years.
I am sure there is a complicated way to explain away my problems with
the BBT but if you could possibly tone it down to a level that a lay
person could understand I would be very grateful
.
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| User: "OG" |
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| Title: Re: My lack of understanding of the Big Bang Theory |
18 May 2007 05:02:53 PM |
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"postman" <postman@mcgarrydware.spam.plus.com> wrote in message
news:464e0eaa$0$8742$ed2619ec@ptn-nntp-reader02.plus.net...
As a lay person I find it very difficult to understand the big bang
theory.
1: if you point the Hubble space telescope at a point in space and leave
it for long enough it will eventually gather enough light to show a galaxy
that is perhaps 12 billion light years away.
If you then swing the telescope 180% in almost anyway plane and repeat the
procedure you will again see a galaxy 12 billion light years away.
These two galaxies are apparently 24 billion light years apart but 12
billion years ago the universe wasn't 24 billion light years across
according to the BBT.
The large scale geometry of the universe isn't difficult, but it isn't
necessarily what we think we are used to. Since separation is in time as
well as in space, you can't just draw a 'map' on flat paper of the two
galaxies and say one is x distance in 1 direction and the other is x
diatance in the opposite direction and so they must have been 2x apart when
light from each was emitted.
The important concept is the idea of 'scale factor'. This is the increase in
linear separation that has occurred over the lifetime of the universe. If 2
objects were separated by a distance d at some time t ago, they are now
separated by a distance d*f(t), where f(t) is the scale factor. This means
that the 24 billion light years separation 'now' is actually the value of
d*f(t)
In effect, you could view the function f(t) as representing the map folded
back on itself - the lines from the 2 galaxies are definitely in opposite
directions, but the separation between them was 'less' in the past - despite
which it took 12 billion years for the light to reach us, half way between
them.
2: Cosmic Background Radiation .. My limited understanding of radiation is
that it starts with an event, consists of photons, travels in straight
lines and travels at the speed of light. It does not sit in the
background. The event happened approx 14 billion years ago and unless we
have been travelling a lot faster than the speed of light for the past 14
billion years we should not be experiencing this radiation. Even if we
have travelled faster than the speed of light due to expansion then the
radiation would also have been speeded up due to that expansion.
It seems to me that you are imagining the BB as an event that happened like
an explosion in a concert hall and we are seeing the explosion as something
that has 'just now' filled the hall with the fastest bits of the explosion
'outside us', and the slower bits 'inside us'. If you play the 'film of the
BB' backwards you would see all the bits of the explosion moving inwards
leaving empty space in the concert hall .
What you need to think of is the 'film of the BB' played backwards including
the whole concert hall, the whole space is getting bigger - and the
separation of items in is is increasing all the time.
To be honest, I don't think that is an 'accurate' metaphor either, but it is
no worse than the other picture and it gives you an alternative way of
viewing the expansion of the universe that (importantly) rejects the 'there
was a single centre' view.
.
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| User: "postman" |
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| Title: Re: My lack of understanding of the Big Bang Theory |
20 May 2007 02:47:31 PM |
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OG wrote:
"postman" <postman@mcgarrydware.spam.plus.com> wrote in message
news:464e0eaa$0$8742$ed2619ec@ptn-nntp-reader02.plus.net...
As a lay person I find it very difficult to understand the big bang
theory.
1: if you point the Hubble space telescope at a point in space and leave
it for long enough it will eventually gather enough light to show a galaxy
that is perhaps 12 billion light years away.
If you then swing the telescope 180% in almost anyway plane and repeat the
procedure you will again see a galaxy 12 billion light years away.
These two galaxies are apparently 24 billion light years apart but 12
billion years ago the universe wasn't 24 billion light years across
according to the BBT.
The large scale geometry of the universe isn't difficult, but it isn't
necessarily what we think we are used to. Since separation is in time as
well as in space, you can't just draw a 'map' on flat paper of the two
galaxies and say one is x distance in 1 direction and the other is x
diatance in the opposite direction and so they must have been 2x apart when
light from each was emitted.
The important concept is the idea of 'scale factor'. This is the increase in
linear separation that has occurred over the lifetime of the universe. If 2
objects were separated by a distance d at some time t ago, they are now
separated by a distance d*f(t), where f(t) is the scale factor. This means
that the 24 billion light years separation 'now' is actually the value of
d*f(t)
In effect, you could view the function f(t) as representing the map folded
back on itself - the lines from the 2 galaxies are definitely in opposite
directions, but the separation between them was 'less' in the past - despite
which it took 12 billion years for the light to reach us, half way between
them.
I cut out the bit about the CMBR because I am still totally confused about
something that starts in one point but doesn't have a centre.
But leaving that to one side for the moment. The two galaxies problem I
thought I understood for a while. I thought of an experiment where I had
an imaginary time machine with my two telescopes on board. I then went
back in space 5 billion years with the two telescopes trained on my two
galaxies with all the necessary gear on board to work out the distances.
When I got to 5 billion years in the past and I measured the distances
to the two galaxies I assumed that they would be 7 billion light years
away each giving a separation of 14 billion light years so that even
though I am measuring the separation of the galaxies at the same point
in time for them they now appear to be 14 billion light years apart
instead of 24 billion light years.
That was fine and I thought I understood that until I thought if they
are 7 billion light years away 5 billion years in the past that means
that the same photon has been falling on earth for 5 billion years and
that can't be right.
At first I thought they must have been closer in the past say 5 billion
light years away. That would give a separation of 10 billion light years
for the two galaxies and that still seemed to make sense. But then I
added up the years. I was 5 billion years in the past and I was looking
at the galaxy a further 5 billion years in the past from there meaning I
was seeing that galaxy 2 billion years further on than I was able to see
it in the present. In other words today when we looked at the galaxy we
were seeing it getting younger and would eventually see its birth. Now
that couldn't be right..
Ok so that must mean that the distance must be greater than 7 billion
light years but by how much. If they were 10 billion light years away 5
billion years in the past then that would have been 1 billion years
before the big bang. If that is the case then the upper limit is 8
billion light years given that galaxies may have taken up to 1 billion
years to form after the big bang. Does this mean that 1 billion years
worth of photons from this galaxy has taken 5 billion years to fall on
earth. This would mean that we are seeing the galaxy in slow motion.
Given all of the above I have obviously gone horribly wrong somewhere
just not sure where. The harder I think about the big bang the less I
understand it. sorry for being so thick..
.
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| User: "OG" |
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| Title: Re: My lack of understanding of the Big Bang Theory |
20 May 2007 04:35:51 PM |
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"postman" <postman@mcgarrydware.spam.plus.com> wrote in message
news:4650a5d4$0$8726$ed2619ec@ptn-nntp-reader02.plus.net...
OG wrote:
"postman" <postman@mcgarrydware.spam.plus.com> wrote in message
news:464e0eaa$0$8742$ed2619ec@ptn-nntp-reader02.plus.net...
As a lay person I find it very difficult to understand the big bang
theory.
1: if you point the Hubble space telescope at a point in space and leave
it for long enough it will eventually gather enough light to show a
galaxy that is perhaps 12 billion light years away.
If you then swing the telescope 180% in almost anyway plane and repeat
the procedure you will again see a galaxy 12 billion light years away.
These two galaxies are apparently 24 billion light years apart but 12
billion years ago the universe wasn't 24 billion light years across
according to the BBT.
The large scale geometry of the universe isn't difficult, but it isn't
necessarily what we think we are used to. Since separation is in time as
well as in space, you can't just draw a 'map' on flat paper of the two
galaxies and say one is x distance in 1 direction and the other is x
diatance in the opposite direction and so they must have been 2x apart
when light from each was emitted.
The important concept is the idea of 'scale factor'. This is the increase
in linear separation that has occurred over the lifetime of the universe.
If 2 objects were separated by a distance d at some time t ago, they are
now separated by a distance d*f(t), where f(t) is the scale factor. This
means that the 24 billion light years separation 'now' is actually the
value of d*f(t)
In effect, you could view the function f(t) as representing the map
folded back on itself - the lines from the 2 galaxies are definitely in
opposite directions, but the separation between them was 'less' in the
past - despite which it took 12 billion years for the light to reach us,
half way between them.
I cut out the bit about the CMBR because I am still totally confused about
something that starts in one point but doesn't have a centre.
But leaving that to one side for the moment. The two galaxies problem I
thought I understood for a while. I thought of an experiment where I had
an imaginary time machine with my two telescopes on board. I then went
back in space 5 billion years with the two telescopes trained on my two
galaxies with all the necessary gear on board to work out the distances.
When I got to 5 billion years in the past and I measured the distances to
the two galaxies I assumed that they would be 7 billion light years away
each giving a separation of 14 billion light years so that even though I
am measuring the separation of the galaxies at the same point in time for
them they now appear to be 14 billion light years apart instead of 24
billion light years.
That was fine and I thought I understood that until I thought if they are
7 billion light years away 5 billion years in the past that means that the
same photon has been falling on earth for 5 billion years and that can't
be right.
At first I thought they must have been closer in the past say 5 billion
light years away. That would give a separation of 10 billion light years
for the two galaxies and that still seemed to make sense. But then I added
up the years. I was 5 billion years in the past and I was looking at the
galaxy a further 5 billion years in the past from there meaning I was
seeing that galaxy 2 billion years further on than I was able to see it in
the present. In other words today when we looked at the galaxy we were
seeing it getting younger and would eventually see its birth. Now that
couldn't be right..
Ok so that must mean that the distance must be greater than 7 billion
light years but by how much. If they were 10 billion light years away 5
billion years in the past then that would have been 1 billion years before
the big bang. If that is the case then the upper limit is 8 billion light
years given that galaxies may have taken up to 1 billion years to form
after the big bang. Does this mean that 1 billion years worth of photons
from this galaxy has taken 5 billion years to fall on earth. This would
mean that we are seeing the galaxy in slow motion.
Given all of the above I have obviously gone horribly wrong somewhere just
not sure where. The harder I think about the big bang the less I
understand it. sorry for being so thick..
I'm sure you're not being thick, trying to work it out from first principles
isn't easy - to be honest I'm not sure what the right answer is either.
However, I suspect it is something to do with the fact that what we are
seeing 'now' is those galaxies as they were 12 billion year ago - i.e. when
they were 1.7 billion years old. If we were to go 5 billion years back in
time, we could not see those galaxies as the light from them would not have
started reaching us.
.
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| User: "postman" |
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| Title: Re: My lack of understanding of the Big Bang Theory |
22 May 2007 04:15:10 PM |
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OG wrote:
"postman" <postman@mcgarrydware.spam.plus.com> wrote in message
news:4650a5d4$0$8726$ed2619ec@ptn-nntp-reader02.plus.net...
OG wrote:
"postman" <postman@mcgarrydware.spam.plus.com> wrote in message
news:464e0eaa$0$8742$ed2619ec@ptn-nntp-reader02.plus.net...
As a lay person I find it very difficult to understand the big bang
theory.
1: if you point the Hubble space telescope at a point in space and leave
it for long enough it will eventually gather enough light to show a
galaxy that is perhaps 12 billion light years away.
If you then swing the telescope 180% in almost anyway plane and repeat
the procedure you will again see a galaxy 12 billion light years away.
These two galaxies are apparently 24 billion light years apart but 12
billion years ago the universe wasn't 24 billion light years across
according to the BBT.
The large scale geometry of the universe isn't difficult, but it isn't
necessarily what we think we are used to. Since separation is in time as
well as in space, you can't just draw a 'map' on flat paper of the two
galaxies and say one is x distance in 1 direction and the other is x
diatance in the opposite direction and so they must have been 2x apart
when light from each was emitted.
The important concept is the idea of 'scale factor'. This is the increase
in linear separation that has occurred over the lifetime of the universe.
If 2 objects were separated by a distance d at some time t ago, they are
now separated by a distance d*f(t), where f(t) is the scale factor. This
means that the 24 billion light years separation 'now' is actually the
value of d*f(t)
In effect, you could view the function f(t) as representing the map
folded back on itself - the lines from the 2 galaxies are definitely in
opposite directions, but the separation between them was 'less' in the
past - despite which it took 12 billion years for the light to reach us,
half way between them.
I cut out the bit about the CMBR because I am still totally confused about
something that starts in one point but doesn't have a centre.
But leaving that to one side for the moment. The two galaxies problem I
thought I understood for a while. I thought of an experiment where I had
an imaginary time machine with my two telescopes on board. I then went
back in space 5 billion years with the two telescopes trained on my two
galaxies with all the necessary gear on board to work out the distances.
When I got to 5 billion years in the past and I measured the distances to
the two galaxies I assumed that they would be 7 billion light years away
each giving a separation of 14 billion light years so that even though I
am measuring the separation of the galaxies at the same point in time for
them they now appear to be 14 billion light years apart instead of 24
billion light years.
That was fine and I thought I understood that until I thought if they are
7 billion light years away 5 billion years in the past that means that the
same photon has been falling on earth for 5 billion years and that can't
be right.
At first I thought they must have been closer in the past say 5 billion
light years away. That would give a separation of 10 billion light years
for the two galaxies and that still seemed to make sense. But then I added
up the years. I was 5 billion years in the past and I was looking at the
galaxy a further 5 billion years in the past from there meaning I was
seeing that galaxy 2 billion years further on than I was able to see it in
the present. In other words today when we looked at the galaxy we were
seeing it getting younger and would eventually see its birth. Now that
couldn't be right..
Ok so that must mean that the distance must be greater than 7 billion
light years but by how much. If they were 10 billion light years away 5
billion years in the past then that would have been 1 billion years before
the big bang. If that is the case then the upper limit is 8 billion light
years given that galaxies may have taken up to 1 billion years to form
after the big bang. Does this mean that 1 billion years worth of photons
from this galaxy has taken 5 billion years to fall on earth. This would
mean that we are seeing the galaxy in slow motion.
Given all of the above I have obviously gone horribly wrong somewhere just
not sure where. The harder I think about the big bang the less I
understand it. sorry for being so thick..
I'm sure you're not being thick, trying to work it out from first principles
isn't easy - to be honest I'm not sure what the right answer is either.
However, I suspect it is something to do with the fact that what we are
seeing 'now' is those galaxies as they were 12 billion year ago - i.e. when
they were 1.7 billion years old. If we were to go 5 billion years back in
time, we could not see those galaxies as the light from them would not have
started reaching us.
I am not sure that could be right either. That would be like saying that
something that was once quite close to us was invisible but now that it
has travelled a long way it suddenly becomes visible again. Imagine a
space ship setting off on a long journey from earth. At first we would
be able to see it quite clearly but eventually it would disappear from
view. Are you saying that if we wait long enough we will be able to see
it again..
Maybe the galaxies aren't moving directly away from us, but I am not
sure if that is allowed in the Big Bang Theory. I am sure it is
something quite simple that I am missing..
.
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| User: "OG" |
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| Title: Re: My lack of understanding of the Big Bang Theory |
22 May 2007 06:16:43 PM |
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"postman" <postman@mcgarrydware.spam.plus.com> wrote in message
news:46535d5f$0$8754$ed2619ec@ptn-nntp-reader02.plus.net...
OG wrote:
"postman" <postman@mcgarrydware.spam.plus.com> wrote in message I'm sure
you're not being thick, trying to work it out from first principles isn't
easy - to be honest I'm not sure what the right answer is either.
However, I suspect it is something to do with the fact that what we are
seeing 'now' is those galaxies as they were 12 billion year ago - i.e.
when they were 1.7 billion years old. If we were to go 5 billion years
back in time, we could not see those galaxies as the light from them
would not have started reaching us.
I am not sure that could be right either. That would be like saying that
something that was once quite close to us was invisible but now that it
has travelled a long way it suddenly becomes visible again.
Ah no, even though the two galaxies 'were' close together they were younger
than the universe and there hadn't been enough time for any light to have
travelled between them.
Imagine a space ship setting off on a long journey from earth. At first we
would be able to see it quite clearly but eventually it would disappear
from view. Are you saying that if we wait long enough we will be able to
see it again..
That's not the same thing because it suggests you can out-run light. What
I'm referring to is that the fact that our galaxy had, in effect, a head
start and is waiting for the light from the other one to catch up with us.
If our galaxy and the other galaxy were 1/2 billion light years apart when
they started emitting light, we wouldn't see each other for another 1/2
billion years if we were in a stationary universe. However the universe is
actually expanding and during the time it takes for the light to go between
us, the universe has expanded by a significant factor ( i.e. it can take a
lot lot more than 1/2 billion years - how long exactly depends on the age of
the universe when the light started being emitted). It's as though there is
a circle around us within which we can see distant things in the past, and
as time passes, that circle increases so that we can see more distant
objects that we couldn't see before.
I think it's probably time to look up some books on the subject. For both of
us probably.
.
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| User: "G=EMC^2 Glazier" |
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| Title: Re: My lack of understanding of the Big Bang Theory |
19 May 2007 08:37:07 AM |
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Postman The BB took place 22 billion years ago. That 13.5 billion age
is one of humankind worse measurements.It was even more worse 100 years
ago. Bert
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| User: "Sam Wormley" |
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| Title: Re: My lack of understanding of the Big Bang Theory |
20 May 2007 01:18:56 AM |
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G=EMC^2 Glazier wrote:
Postman The BB took place 22 billion years ago. That 13.5 billion age
is one of humankind worse measurements.It was even more worse 100 years
ago. Bert
Actually there are four independent measurements, Herb, that give
roughly the same age for the universe.
o supernovae data (14.3 billion years)
o Sloan Digital Sky Survey (13.6 billion years)
o Hubble expansion data 72 km/sec/Mpc (13.6 billion years)
o CMB data 71 +/- 4 km/sec/Mpc (13.7 billion years)
What is the evidence for your 22 billion year figure?
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| User: "Sam Wormley" |
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| Title: Re: My lack of understanding of the Big Bang Theory |
18 May 2007 04:43:22 PM |
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postman wrote:
As a lay person I find it very difficult to understand the big bang theory.
1: if you point the Hubble space telescope at a point in space and leave
it for long enough it will eventually gather enough light to show a
galaxy that is perhaps 12 billion light years away.
If you then swing the telescope 180% in almost anyway plane and repeat
the procedure you will again see a galaxy 12 billion light years away.
These two galaxies are apparently 24 billion light years apart but 12
billion years ago the universe wasn't 24 billion light years across
according to the BBT.
2: Cosmic Background Radiation .. My limited understanding of radiation
is that it starts with an event, consists of photons, travels in
straight lines and travels at the speed of light. It does not sit in the
background. The event happened approx 14 billion years ago and unless we
have been travelling a lot faster than the speed of light for the past
14 billion years we should not be experiencing this radiation. Even if
we have travelled faster than the speed of light due to expansion then
the radiation would also have been speeded up due to that expansion. If
my admittedly limited understanding is correct the event that created
the CMBR actually happened quite some time before the galaxies were
formed. So the CMBR would have passed the position the galaxies were to
form in before they were even born. If the CMBR bounced of the edge of
the universe then does all radiation bounce of the edge of the universe
and if so should we be seeing mirror images of galaxies whose light has
been bounced in a similar way. If it does bounce how many times will it
have bounced in 14 billion years.
I am sure there is a complicated way to explain away my problems with
the BBT but if you could possibly tone it down to a level that a lay
person could understand I would be very grateful
These will help...
No Center
http://www.astro.ucla.edu/~wright/nocenter.html
http://www.astro.ucla.edu/~wright/infpoint.html
Also see Ned Wright's Cosmology Tutorial
http://www.astro.ucla.edu/~wright/cosmolog.htm
http://www.astro.ucla.edu/~wright/cosmology_faq.html
WMAP: Foundations of the Big Bang theory
http://map.gsfc.nasa.gov/m_uni.html
WMAP: Tests of Big Bang Cosmology
http://map.gsfc.nasa.gov/m_uni/uni_101bbtest.html
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| User: "Ben Rudiak-Gould" |
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| Title: Re: My lack of understanding of the Big Bang Theory |
18 May 2007 04:02:38 PM |
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postman wrote:
As a lay person I find it very difficult to understand the big bang theory. [...]
Ned Wright's cosmology tutorial is a fantastic introduction to this subject:
http://www.astro.ucla.edu/~wright/cosmolog.htm
Read that first, and if you're still confused then post again here.
-- Ben
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