Cosmology Help

[gth759k]

I recently sent an email to several physics professors at CalTech and MIT with a question I have about Cosmology and only one professor (Walter Lewin) replied simply saying good luck! Its not so much of a question as it is a problem. I've talked to people about this before and the conversation quickly looses all basis in reality and data and becomes more like something out of a science fiction novel. So, if you should choose to reply please stick to the facts and leave out the suppositions. That means I don't want to hear anything like stretchy space-time unless you can back it up with systematic, uniform and precise data and theory, because you'll completely lose my respect. So, on to the problem.

According to many sources (on the web and off) ... the universe is accelerating! So I set about searching for the evidence of this claim and I was pointed to many sites on redshift and spectography and physics equations. Eventually I found this image

Click to view full size image

which is most interesting to me. I don't know about you, but I can't think in megaparsecs, so the first thing I did was convert the units from megaparsecs to kilometers. The closest type 1a nebula in the lower left hand corner is about 35 Mpc = 1.0799 x (10^21) km and the farthest nebula in the upper right hand corner of the image is about 635 Mpc = 1.9594 x (10^22) km. These values on the horizontal axis were approximated, as I understand using the magnitude-distance formula which uses the absolute and apparent magnitudes to determine the distance. The vertical axis on the chart shows the (approximate) velocity in km/s of the nebula relative to us calculated using Hubble's law. Now, let me ask you a question, what does this chart show us? I've been told that this chart shows that the farther away a celestial body is the faster its going! Hmmm ..... I disagree. I think this chart shows that the farther away a celestial object is, the faster it WAS going. I say that because, and I'm sure you'll agree, we see the galaxies and nebula as they WERE and not as they are. So according to the data, the farther away you look, the faster things appear to be moving (relatively) away from us. When we look at the most distant objects we are seeing a picture of the early universe right? Wait, so doesn't that mean that the fastest objects we find are a picture of the early universe? And the closer, more resent, (slower) objects are a picture of a more recent universe? Then the velocities of the closest galaxies and nebula are a better representation of what the universe is like right now because their data is the most recent. It looks like to me, if you reconstruct the events according to the data, that the early universe was moving away from us faster than the more resent universe. Just like an explosion! Wait, explosions don't accelerate.

[Chris Peterson]

I recently sent an email to several physics professors at CalTech...

That's Caltech.

So, if you should choose to reply please stick to the facts and leave out the suppositions. That means I don't want to hear anything like stretchy space-time unless you can back it up with systematic, uniform and precise data and theory, because you'll completely lose my respect.

The idea that space-time expands is mainstream cosmology. If you are entering into this thinking that concept is weak, you're headed for problems.

According to many sources (on the web and off) ... the universe is accelerating!

You might be confusing a couple of ideas here. We think that the Universe is expanding. What that means is that the distance between distant points is increasing with time. That idea has been around a long time, and is very well supported by theory and observation. Until recently, it was not clear if the rate of expansion was steady, decreasing, increasing. It now appears that it is increasing, which means the Universe will continue expanding forever. That is a sort of acceleration, and maybe is what you are referring to. But it is different from simple expansion.

Now, let me ask you a question, what does this chart show us? I've been told that this chart shows that the farther away a celestial body is the faster its going! Hmmm ..... I disagree. I think this chart shows that the farther away a celestial object is, the faster it WAS going.

Really, what this chart is showing is how much space has expanded between us and the object since it emitted the photons we are measuring. It is a common mistake to treat redshift as if it were Doppler shift, the product of a recessional velocity. What we are observing is the wavelength shift that occurred over a long time (frequently, billions of years) as a photon moved through expanding space. When the photon was emitted, the object was much closer to us, and perhaps not moving very fast with respect to us. But because space was expanding, it took a very long time for that photon to finally reach us, and during that time its wavelength was shifted longer. That really does mean that we are looking a measure of the current recessional velocity, not the original.

[gth759k]

It is a common mistake to treat redshift as if it were Doppler shift.

Not according to http://hyperphysics.phy-astr.gsu.edu/hb ... edshf.html.

I just want to shift the whole idea back into reality. For instance, you're claiming that it was the photon's journey, through the vacuum of space and time, elongating the electromagnetic wave, that shifted the wavelength. How do you know? We can both agree that redshift is defined as the wavelength observed minus the wavelength emitted divided by the wavelength emitted. Right? We can both agree that distant galaxies were once closer together than they are now right? Many people I've talked to, including you, seem to skip the fundamentals and go strait from facts like galaxies moving apart to the supernatural like dark matter. Which doesn't help me.

[Chris Peterson]

It is a common mistake to treat redshift as if it were Doppler shift.

Not according to http://hyperphysics.phy-astr.gsu.edu/hb ... edshf.html.

That source is wrong. There is such a thing as Doppler redshift (and blueshift) and it is used to determine relative motion between objects that are close to one another. Examples include the relative motion of stars in a galaxy, or the rotation characteristics of galaxies.

Cosmological redshift is not the same as Doppler redshift. Cosmological redshift is caused by the expansion of space as the light was in transit. That can be used to estimate the current recessional velocity, but the math is different, and the mechanism is different.

I just want to shift the whole idea back into reality. For instance, you're claiming that it was the photon's journey, through the vacuum of space and time, elongating the electromagnetic wave, that shifted the wavelength. How do you know? We can both agree that redshift is defined as the wavelength observed minus the wavelength emitted divided by the wavelength emitted. Right? We can both agree that distant galaxies were once closer together than they are now right? Many people I've talked to, including you, seem to skip the fundamentals and go strait from facts like galaxies moving apart to the supernatural like dark matter. Which doesn't help me.

I have a suspicion that you aren't really looking for help here.

[gth759k]

Cosmological redshift is not the same as Doppler redshift. Cosmological redshift is caused by the expansion of space as the light was in transit. That can be used to estimate the current recessional velocity, but the math is different, and the mechanism is different.

How do you separate the different forms of shift on a spectrograph? How do you know that the observed redshift isn't a summation of the Doppler and gravitational and cosmological shifts? In which case a relative velocity could create an identical redshift as a gravitational field with a very particular mass. If two galaxies were not moving relative to each other, and space wasn't expanding, a difference between the masses of the galaxies could still create a shift. Very large shifts like z > 5 can be produced mathematically by either relative velocity or gravitational fields. For example, a wave of light is red shifted as it leaves a galaxy, due only to its mass. If the universe were more dense in some locations than it is in others, which it is, as light leaves those areas it can become red shifted due only to the mass of the dense regions it left behind. I'm looking for answers yes, but everyone thinks that its all already been solved and they know all the answers; they don't.

[Guest]

http://en.wikipedia.org/wiki/Redshift

[Chris Peterson]

How do you separate the different forms of shift on a spectrograph? How do you know that the observed redshift isn't a summation of the Doppler and gravitational and cosmological shifts?

From a purely observational standpoint, you can't separate Dopper, cosmological, and gravitational redshifts. All contribute to the shift seen is spectra. But that doesn't mean that we can't make reasonable assumptions about what we see. There is no good physical basis for recognizing the redshift of distant objects as Doppler, but there is a good basis (General Relativity) for interpreting the redshift as cosmological. The relationship between distance and redshift is very well predicted by GR, and GR has many independent verifications, meaning that we have a high degree of confidence it accurately describes the Universe.

In order to attribute the observed redshift to Doppler mechanisms, we would need a different set of theories describing the Universe. So while it can't be proven that the redshift of distant objects is cosmological, that is by far the most rational interpretation based on our current understanding of physics.

[VoijaRisa]

There is no good physical basis for recognizing the redshift of distant objects as Doppler, but there is a good basis (General Relativity) for interpreting the redshift as cosmological. The relationship between distance and redshift is very well predicted by GR, and GR has many independent verifications, meaning that we have a high degree of confidence it accurately describes the Universe.

In order to attribute the observed redshift to Doppler mechanisms, we would need a different set of theories describing the Universe. So while it can't be proven that the redshift of distant objects is cosmological, that is by far the most rational interpretation based on our current understanding of physics.

To expand on this:
If we were to interpret the observed redshifts as Doppler, it would mean that numerous galaxies were far exceeding the speed of light. This makes no sense, and thus, can be effectively ruled out.

If we were to interpret the observed redshifts as Gravitational, it would mean that the mass of galaxies were orders of magnitude more massive than observed and also a function of distance. This makes no sense, and thus, can be effectively ruled out.

If we were to interpret the observed redshifts as Cosmological, this fits perfectly with relativity which predicts the expansion of the universe and beautifully explains the redshift being a function of distance. The harmony of this model with observations and predictions is why it is accepted.

[jman]

These are great explanations, thanks.