Starship Asterisk*

I admit to having no expertise but...the galaxies farthest away are receding faster than the galaxies that are nearer. The galaxies farther away are further back in time and the nearer galaxies are closer to the present time. Since the galaxies nearer to the present time are receding slower than the galaxies further from the present time, it seems to me then that the the expansion of the universe is slowing. Where am I going wrong?
Thanks for your patience.
Ed Kelly

I am not the best person to ask about this, but as for the acceleration of the universe, this is how I understand it.

In the late 1990s, there were two teams who used supernovas type Ia to measure the expansion of the universe. Supernovas type Ia are supposed to always be about equally bright, and therefore they can be used as "standard candles". Supernovas type Ia are characterized by their light curves. You can recognize a supernova type Ia by how it first rises and then falls off in brightness in a characteristic way. Since these supernovas are always more or less equally bright, you can measure the apparent brightness of the distant supernova and figure out how much its light has been dimmed by its distance from us, and therefore, you can tell from its apparent brightness how far away it really is.

Astronomers use cosmological redshift to understand how much the universe has expanded since the light of a certain light source was emitted. But the redshift does not say how far away the light source was when it emitted its light into space.

A distant supernova type Ia will be redshifted, and astronomers can use its redshift to figure out how much space has expanded since the supernova exploded. But they can't tell from the redshift how much space had expanded before the supernova exploded.

If space had expanded a lot before the supernova exploded, and has since slowed down, then the supernova "will have had a head start" that carried it moderately close to us (well, comparatively speaking). The supernova will then be comparatively bright. This would be a sign that the universe was expanding faster at earlier epochs than it is doing now, and that it is slowing down. (Note: One single supernova can't tell astronomers very much at all. They need many supernovas so that they can talk about averages.)

All right. If the universe expanded relatively slowly at first, and then faster and faster, then the slow initial expansion will not have "carried the star that would become the supernova" very far at the time when it exploded. The supernova will be farther away than astronomers had expected, and it will therefore be comparatively faint.

This is what astronomers found. The distant supernovas type Ia are fainter than you would expect, so they seem to be farther away than they should be if the universe had expanded at a constant rate - and much farther away than they should be if the universe was actually slowing down.

There are also other signs that the universe is speeding up, but I'll leave that to others to talk about.

Ann

Ed Kelly wrote:I admit to having no expertise but...the galaxies farthest away are receding faster than the galaxies that are nearer. The galaxies farther away are further back in time and the nearer galaxies are closer to the present time. Since the galaxies nearer to the present time are receding slower than the galaxies further from the present time, it seems to me then that the the expansion of the universe is slowing. Where am I going wrong?

First of all, it's not really accurate to say that any galaxies are moving away from us (outside of their actual relative motion, which is as likely to be towards us as away). What's happening is that space is expanding (fairly) uniformly, which stretches out the wavelength of light and produces redshift. The closer something is, the less the intervening space as expanded and therefore the lower the redshift.

Second, galaxies far away aren't exactly further back in time. We're simply seeing the photons that were emitted long ago. The objects themselves are the same age as the rest of the Universe (less the time of their formation) and are actually much farther away now than they appear to be when we observe those old photons. But because the photons were emitted long ago, they have covered more distance and experienced greater redshift. The oldest photons we see are the ones comprising the cosmic microwave background, which have shifted from the visible (or shorter) clear into the microwave spectrum. Those photons have been traveling nearly 13.7 billion years before we detect them.

Ann has nicely described the reasoning behind the theory that the actual rate of expansion isn't constant, but for the last few billion years has been slowly increasing.

Ed Kelly wrote:I admit to having no expertise but...the galaxies farthest away are receding faster than the galaxies that are nearer. The galaxies farther away are further back in time and the nearer galaxies are closer to the present time. Since the galaxies nearer to the present time are receding slower than the galaxies further from the present time, it seems to me then that the the expansion of the universe is slowing. Where am I going wrong?
Thanks for your patience.
Ed Kelly

According to Hubble´s law, it follows that the most
distant galaxies traveling faster. A more away from us,
more speed.
Hubble established a constant for that away wich is
at 70 km/sec/Mpcs.

saturno2 wrote:According to Hubble´s law, it follows that the most
distant galaxies traveling faster. A more away from us,
more speed.
Hubble established a constant for that away wich is
at 70 km/sec/Mpcs.

Except that the universal expansion rate is now known to not be a constant, because the rate of expansion is accelerating.

To me the two greatest cosmological discoveries of the last century where (1) The expansion discovered by Hubble and others, implying a beginning to our Universe in the Big Bang and (2) The discovery of the current acceleration of the expansion rate, implying the existence of Dark Energy and that there will never be a Big Crunch.

Thanks to all who replied. I will be printing your responses and reading them over and over to sort this out. A little more research is also warranted.
Thanks again,
Ed

Ed Kelly wrote:Thanks to all who replied. I will be printing your responses and reading them over and over to sort this out. A little more research is also warranted.
Thanks again,
Ed

Hello Ed - I wonder if you'd find this link helpful? The European Association for Astronomy Education organised a teacher training course each year for several years to improve and promote scientific astronomical education in European schools and published detailed teaching material on its website.

In 2004 the subject of the Summer School was

HOW DO WE KNOW THAT THE UNIVERSE IS EXPANDING?
By the end of the workshop you should be able to:

• State the definitions of magnitude, the parsec and the light-year.
• Calculate the distance of nearby stars given their annual parallax.
• Calculate the relative distances of stars using the 'inverse square law'.
• Explain how Henrietta Leavitt discovered that Cepheid variable stars can be used to measure the distances of stars and nearby galaxies.
• Explain the importance of Type Ia Supernovae for measuring the distances of very distant galaxies.
• Measure the velocity of a galaxy from its spectrum.
• Understand that for distant galaxies, Edwin Hubble discovered the link between velocity and distance.
• Explain how better observational techniques caused the value of the Hubble Constant to fall from 530 to 72 km/s Mpc in 80 years!
• Explain how the value of the Hubble Constant at great distances could predict the fate of the Universe.


http://skolor.nacka.se/samskolan/eaae/s ... ubble.html

I've found the material on the website to be useful, even though one doesn't have the benefit of the teacher who presented the material at the workshop.
Best wishes
Margarita

Margarita,
Great website. Thank you very much.
Ed