D-16: Postcards of a Bygone Age

The following was written by Murmeldjuret on the Stellaris forums. Thanks, Murmeldjuret.


Today is the seventh anniversary of something that at first glance might appear unimportant. A gamma ray burst appeared in the sky. This happens regularly, but this one was special. For being a gamma ray burst it had a lower frequency for the shape and strength it had. Almost everything that reaches us is redshifted, and this one was clearly so. It had a redshift of z=8.2, which is probably just a number to you, but it makes it the third most distant object we know of. The gamma ray burst happened over 13 billion years ago. The universe was only 600 million years old when the dying star created the gamma ray burst. GRB090423 kept the record for oldest thing we have observed until last year, and is still the oldest thing from which we a spectrographic readout.

Last year we saw this galaxy:

Spoiler: GN-z11

Enter a caption

The galaxy GN-z11 has a redshift of z=11. Making it a postcard from 13.4 billion years ago. Wedetermined only last month that is twenty five times as small and has 1% the mass of the Milky Way. Despite that it forms stars at least 20 times as frequent. It shows that the universe was a bit different 13.4 billion years ago. It might be a typical early galaxy, a typical protogalaxy, or possibly even the first galaxy. It is after all only 400 million years younger than the UNIVERSE. For comparison it was 540ish million years ago that the first land life came. Currently whatever is left of it is a staggering 32 billion lightyears away since the universe has expanded during the 13.4 billion years it took its light to reach us.

Sadly, this might be the furthest back we can see galaxies for several reasons. Beyond this might be no things bright enough to see, and a bit further back is a wall of light that covers all of the then existing universe. This is the cosmic microwave background. It was the discovery of the cosmic microwave background that made the Big Bang the dominant theory for creation of the universe. After a rapid inflation for a few thousand years after Big Bang it would still be a shining ball of light, and it is this light, now redshifted to microwave, that is thebackground. Space is not absolute zero but a few Kelvin above it because of this background radiation.

Spoiler: Timeline of Universe from NASA

Spoiler: Microwave background from WMAP


Almost every galaxy is redshifted, and since the distance between us increasing, we are fairly confident the universe is expanding, and has been for a long time. In fact, the velocity we are expanding with is proportional to the distance, and is termed the Hubble Constant, from the Hubble Law. Constant is not quite a correct term, we now know the constant varies with time. Up until a few billion years ago the “constant” was decreasing. The universe had been slowing down with its expansion. A few billion years ago however, it began increasing, and it is still accelerating. The universe is becoming larger, and the rate at which it is becoming larger is also increasing.

Until 1990 we assumed the cosmological constant or the energy density of vacuum was 0. But with the rate of expansion increasing, it can’t be. The universe is filled with something called dark energy which functions like gravity on a cosmological scale. As part of the Lambda-CDM model it is the current prevailing theory because it best fits the evidence with the least variables for error. Now one has to wonder what the other theories would be when the prevailing one includes something we know nothing about, appears to defy a lot of science, and accounts for 70% of all energy while being invisible. Just 5% of the universe would be visible matter, most of it dark energy, and the rest dark matter. It seems to fit the data well, but the data is only for the past 30 years out of a timeline of 13.9 billion.

Explaining what dark energy is might bridge the gap between quantum mechanics and relativity, give us FTL, and enlighten us on the birth anddeath of the universe(s).
Most likely, it will just raise more questions.


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