Expert: Tom Whiting - 3/4/2010

Question
QUESTION: Sir,
- Hubble's law explains about an expanding of universe everywhere in the PAST or in the PRESENCE? (We can observe the universe in the PAST only?)
- How can we got a "standard spectrum" (spectrum when galaxies are immobile)so we know the redshift spectrum of galaxies?

ANSWER: Hi Khanh,
The "standard spectrum" is the laboratory spectrum that is observed in a laboratory at zero
velocity here on the Earth. A distant galaxies' red shift is compared to the laboratory standard spectrum.  No galaxy is stationary (or immobile)... everything is moving relative to everything else. There is no such thing as absolute rest in the Universe.

Hubble's value of 71 Km/sec per megaparsec is the current value.
It was probably different in the past, and it will definitely be different in the future.
Since the acceleration of the expansion of space is increasing, Hubble's value will increase too.

Yes, we can only see the past, not the present or the future.  But we do know that the universe
is accelerating it's expansion as time goes by. So we will either suffer from a 'big rip'
or a heat death, trillions of years from now.

The Universe is like a loaf of expanding raison bread in the oven... the bread is expanding, but not the raisons (galaxies). But no matter which raison you pick to represent our galaxy, all
the other raisons are receding from each other, and the more distant the other raisons, the faster they move away, proportionally. And the oven heat is also moving upward, so the bread expansion is also increasing speed with the passage of time. So the bread expands faster and faster as time goes by.  That's the correct picture of our Universe.
Hope this helps,
Clear Skies,
Tom Whiting
Erie, PA USA

---------- FOLLOW-UP ----------

QUESTION: Thanks for your help!

You said: "The "standard spectrum" is the laboratory spectrum that is observed in a laboratory at zero velocity here on the Earth"

Do you mean we create an artificial galaxy on Earth?

Answer
Yes, in a way, we do; I guess one can look at it that way. I'm no expert on spectral analysis, but this is the basic idea in the laboratory.  (Recall that light is just different wavelengths of the same photon).
Anytime a substance is heated to incandescence (by burning at a high temperature), that light produces a spectrum, which in the laboratory we pass through a spectrometer and record it's light spectrum (or wavelengths) at rest.  If you burn all the elements and combine all the light, you then have a "standard spectrum" at rest, or zero velocity. This is called a continuous spectrum. (There are also bright line spectrums and absorption spectrums too, but you can google those).

The light from the galaxy is also passed through a spectrometer and then we have it's spectrum,
especially the double lines of sodium light with are most prominent.
Scientists observe the sodium line wavelengths on their laboratory spectrum, at rest (from burning sodium), and again from a very distant galaxy receding at a very high speed. These double lines of sodium from the receding galaxy will be shifted a certain amount toward the red end, based solely on their speed of recession, and from that we can deduce the speed of recession and thus the distance based on the Hubble value. (The shift in wavelength is identical to a Doppler shift with sound where the tone (wavelength) changes as the "race car" or train whistle goes past you at high speed.)  With a source of moving light it's simply a wavelength shift also...so they are basically measuring the doppler shift of light, like we do for sound, or doppler radar is another example. The same Doppler law, a shift in wavelength due to the relative velocities... that's been around for years.  It's really straight forward in practice.

So in summation, a galaxy makes a spectrum with the hot gases in it's stars giving off radiation.
We do it in the laboratory by incandescent burning of the elements at rest, and a comparison
is observed. And since a hot atom of sodium on Earth is the same as a hot atom of sodium in a galaxy, (and all the other elements are identical no matter where you are in the Universe)
we can then measure the shift in the wavelength of light. This is called 'red shift' because
most galaxies are receding from us. A few nearby galaxies are approaching (a blue shift of wavelengths), like Andromeda, but these are just our "Local Group" galaxies whose local velocity and gravity can easily over-ride the general expansion of the space-time continuum. But those are the only galaxies approaching us... the other billions of distant galaxies are all receding,
and receding proportionally... the farther the galaxy, the faster it's receding.  (Edwin Hubble circa 1929.)
Hope this helps,
Clear Skies,
Tom

FOLLOW UP:
Even if one can't burn elements in the lab, we could always use the solar spectrum as a
galaxy standard... the sun is basically at rest relative to the Earth, compared to the high
velocity of the distant galaxies.
Each element, each isotope (extra neutrons in the nucleus), each ionized level of an element (missing electrons), and each compound....has it's own individual unique set of spectral lines.
Just like every human has his own unique fingerprints or DNA...each spectral signature of
each material, is unique.
So with a spectrum, an expert in the field (I'm not, obviously) can tell you which elements are present in the light source, and even how much of each element is present (relative to the other elements), and even whether there is a magnetic field present and it's strength (because a magnetic field will curve the straight lines of the spectrum slightly).
We've come a long way since Edmund Halley first discovered the spectrum of light, which is really
what a rainbow in our sky is displaying... the solar spectrum.
Just thought you'd like to know all this.
Clear Skies,
Tom