Quagmire of Thought

[Merlin63]

I recently posed this question to a physics major at a well known University, I have not yet gotten a reply.

Dear Sir,

Let me start by apologizing for disturbing your day with what may be a silly email.

I'm trying to understand how scientists (cosmologists/astronomers/physics PhD's etc.) say that the universe is expanding more rapidly. Since I don't have a PHD in Physics, could you give me a layman's explanation of how they know this. I've seen the shows on the discovery channel that says they know this by the RED/BLUE shift of light being greater the further an object is from the Earth, but if the light traveling to us is let's say 12 billion years old, isn't therefor, the RED/BLUE shift of light from that distant point in space? I mean, how do we really know how fast that distant galaxy/quasar/supernova or what have you, is going, relative to the Earth, in REAL time to us right now? I don't think we can know, so in my opinion, how can we say that the universe is expanding more rapidly?

I'm caught in a quagmire of thought over this. A simple explanation sure would help my simple mind understand it.

Thank you sincerely for your time reading this and I hope I didn't sound crazy for the way I think of this subject.

Sincerely,

MP

If anyone in the "Stargazers" Forum would care to take this one up I would be greatly appreciative of a reply.

[yeti monster]

I have to confess that it makes not one jot of difference to me personally, although such a thing is handy to know during the pub quiz I suppose.

So long as the stars and galaxies can be placed in front of my scope, I'm content enough to look at them.

[Merlin63]

Thanks for the reply "Yeti" I guess I was just thinking out loud......That's still ok isn't it? Or have they made it a crime these days?

I have too many questions to ask too many people.

[Doc]

Taken from the "Curious about Astronomy" website....

Astronomers measure the movement of objects relative to us using Doppler shift. When you hear a train coming, its whistle is heard at a different frequency compared to when it is receding, right? In the same way, light also has a Doppler shift, whereby its frequency is shifted depending on the motion of the emitting object.

Astronomers observed that light from distant objects in the universe is redshifted (shift in the frequency of light towards red color), which tells us that the objects are all receding away from us. This is true in whatever direction you look at: all the distant galaxies are going away from us. This can only be due to the fact that the Universe is expanding.

Further, by measuring the distance to the galaxies, one finds that the velocity of recession is proportional to the distance of the galaxy from us. This is called Hubble law after Edwin Hubble who was the first to discover it.

Astronomers do not know for sure the value of the Hubble constant. In fact there are many debates going on about that, since it is a very important factor in understanding the history of the universe. There are many estimates done with many different techniques, all of which have sizeable uncertainties associated with them. Generally, we assume the value to be between 50 and 100 km/s/Mpc (which means that for each Megaparsec (Mpc), galaxies appear to be receiding at 50-100 km/s, one Mpc being equivalent to 3.2 million light years)

The techniques used to estimate the Hubble constant involve looking at astronomical objects we understand the behaviour of, and comparing the observations at various distances from us. Some of the objects used are Cepheid variables (young and massive stars whose luminosity change periodically), supernovae (explosion of a star at the end of its life) and globular clusters (dense groups of old stars found in the centre of galaxies). We use these objects because we get to have an estimate of their distances, which is generally very hard to do in astronomy. Knowing the distance and the redshift of these galaxies (i.e. how fast they are moving away), which is relatively easy to get, astronomers can get an estimate of Hubble's constant. There are two groups of people finding two different values. Some believe the Hubble constant is on the order of 55 km/s/Mps, while others find something around 85 km/s/Mpc.

I cannot give you a firm answer on the value of the constant, because there are so many estimates, without much to decide which is the best. Recent analysis of Cepheid variables have given a value of 72+/-8 km/s/Mpc (you can see the large error bars put on that value!), but some people argue that it might be overestimated.

[yeti monster]

Don't take it wrong, "Merlin", what I mean is that, for me, such knowlage won't alter how I live and view the universe, as it will have no direct effect on my life.

It is good to raise these questions if they mean something to you.

An understanding of simple physics is enough for me to get through life, to understand how snatching the steering wheel to the left or right at 90 mph will effect how the car behaves is something I can relate to and make us of. To know the latest theory on the age, size and origins of the universe, well, I can't really see what differnce it would make one way or the other (to the ordinary man in the street).

Obviously you do want to know, I wasn't trying to make out that this is in any way wrong.

I apologise for wasting your time.

[themos]

isn't therefor, the RED/BLUE shift of light from that distant point in space?

I think there could be a theory that does this but it would mean that the "laws of physics" change in time. The scientists would prefer to see if they can explain phenomena without that feature because once you allow change in that you get too many possibilities. Remember, the science game is not just "can I explain this?". It is, rather, "can I explain this in the most economical fashion?"

[johninderby]

There's the speed with which different objects are moving relative to each other AND then there is the expansion of the universe as well so it can get a bit confusing.

There's a pretty good short and simplified explanation here

http://www.esa.int/esaSC/SEM8AAR1VED_index_0.html

John

[Merlin63]

Thanks for the explanation and I have read this before,(The Hubble Constant) however, how do we really know in what direction, or if the speed of the distant object has slowed over time? Maybe galaxies travel in circles and not straight lines? These are questions that burn my mind.

[brianb]

You'll never get this sort of thing until you realize that "now" is a relative term which really doesn't have any meaning except for objects in very close proximity to each other in at most a weak gravitational field. That means it is a useful concept for us earth dwellers so long as we stick to objects close enough to touch, or at least send probes to, though even there the concept is getting wooly.

Suppose I'm watching a live image transmitted from a probe on Mars, at the moment it's 7 light minutes distant. That's "now", right? But if I twist the control wheel to make the camera zoom, and the camera on the probe responds instantly, I don't see the effect on my monitor until 14 minutes have elapsed. "Now" on Mars is 7 minutes in my past, and "now" on Earth is 7 minutes in the probe's past.

It makes no sense to talk about events during that time gap until sufficient time has elapsed to reveal them. All we can do is predict, based on laws of quantum mechanics - which resolve to general relativity over large scales when large amounts of matter at temperatures considerably above absolute zero are concerned.

So far as the expansion of the universe is concerned, we can calculate the acceleration parameter given by Hubble's "constant" from observations. As we observe objects at greater and greater apparent distances, we find that the "constant" is in fact increasing with distance, implying an acceleration of the expansion. We have no reason to suppose that our viewpoint is special and no means of making observations from points widely seperated in time or space from our own, and must therefore rely on the statistics drawn from quantum mechanics to deduce that the effect is universal.

[Merlin63]

Thanks again Yeti, I suppose Newton could have said the same thing about his theories on gravity, but he kept asking himself questions more than he did with other people. I guess that's what I'm doing here, just thinking out loud.

[arad85]

I mean, how do we really know how fast that distant galaxy/quasar/supernova or what have you, is going, relative to the Earth, in REAL time to us right now?

We don't know how fast it is going right now - all we know is how fast it was going however many years ago we can see it from. What we do is assume the laws of physics hold throughout ther universe and we can infer that the galaxy has moved away from us in that time (why would it suddenly stop moving?).

I don't think we can know, so in my opinion, how can we say that the universe is expanding more rapidly?

More rapidly than what? What we can infer is that most of the galaxies are moving away from us - some quicker than others.

If anyone in the "Stargazers" Forum would care to take this one up I would be greatly appreciative of a reply.

Well.. always happy to have an opinion

[themos]

We have no reason to suppose that our viewpoint is special

I think that might need modifying.

[Merlin63]

Thanks for the link on "Red-Shift" but I do understand what it is, I just am kind of cloudy about how they can know or even predict what something 12 billion light-years old (and away from us) is doing "Right Now" that's all I'm saying.

[Merlin63]

ARAD85 What I should have said is "More rapidly over time"

[Merlin63]

I guess I can admit that galaxies were traveling much faster 12 billion years ago, but they talk about how "Dark matter" can effect galaxies and such. How do we know that "Dark matter" can't also slow down the movement of galaxies?

[Merlin63]

OH Well....Just a thought. One thing you wont catch me doing is making 100% for sure statement without all the facts in hand. But that is what science attempts to do. That's the positive point of it all.

[themos]

"Right Now", strictly speaking, makes sense only for the person making the statement.

When you want to extend it to experiences happening far away, Mars say, you can say that "Right Now in Mars" is the event that my telescope is showing "Right Now". Or you can add the light travel time. Or you can say that "Right Now" is so many seconds since the big bang and "Right Now in Mars" is the same amount of seconds. But Einstein says that to do this you have to first agree on some fixed trajectory of the second-counting clock in all the intervening time as different trajectories will show different elapsed times.

Ahve I managed to confuse both of us now?

[Merlin63]

I suppose a person could accept the first answer to any question they pose to somebody but that would only be detrimental to the the REAL answer they seek and then they find out that the world really IS round!

I read that quote somewhere.

[Merlin63]

Just to show you how nutty I am.......I also believe that the speed of light can be broken (I know of an experiment to prove or disprove the theory that hasn't as yet been tried), and that black holes don't warp space/time but only gravity fields.....but we wont go there in this forum.

[Jove]

Possibly mis-remembered and muddled, the line of reasoning is something like:

- we observe supernovae in distant galaxies.

- our study of supernovae in general means we have an idea of how bright they are (absolute magnitude)

- by observing the spectra of the distant supernovae we can determine the redshift of the galaxy in which they are occuring and hence the (radial) velocity of the galaxy.

- from the observation of the observed magnitude of the supernovae and the redshift we can determine the relationship between distance (brightness of the supernovae) and velocity (redshift).

- now, in Hubble's cosmology, this relationship was a stright line, hence the 'Hubble constant', which would be the 'm' in the equation y = mx + c you may recall from school

- it turns out, though, that observations of very high redshift supernovae are brighter than we would expect. I seem to recall that the greater the redshift the greater the discrepancy. This is to say that the supernovae are not as far away as we would expect if expansion were constant - the explanation for this is that the expansion of the universe was slower in the past and hence that the expansion must be accelerating. If the expansion were slowing (faster in the past) then we would expect distant SN (should have discovered that abbreviation sooner!) to be dimmer than we expect.

As to what the expansion is 'now'... that question all depends on where you measure 'now' - relativity which makes my head hurt

[Chris H]

Read 'Big Bang' by Simon Singh. Very good book and will give you answers.

[barkis]

There is a new phenomena been born, "Dark Flow, a programme about which was on TV recently.

I'm sorry I can't put it into sense for you, as most of it was over my head, but the programme may be available on one of the TV iPlayers.

This new 'force', If that is a proper term for it, seems to have a huge influence on matter.

Ron.