Posted on 07/01/2017 7:01:15 PM PDT by ETL
Newsflash: the universe is expanding. We've known that since the pioneering and tireless work of Edwin Hubble about a century ago, and it's kind of a big deal. But before I talk about dark energy and why that's an even bigger deal, I need to clarify what we mean by the word "expanding."
The actual observation that you can do in the comfort of your own home (provided you have access to a sufficiently large telescope and a spectrograph) is that galaxies appear to be receding from our own Milky Way. On average, of course: galaxies aren't simple creatures, and some, like our a-little-too-close-for-comfort neighbor Andromeda, are moving toward us.
This recession is seen in the redshifting of light from those galaxies. The fingerprint frequencies of certain elements are shifted down to lower frequencies, exactly like they are for the Doppler effect. But to explain the cosmological observations as a simple Doppler shift requires a few head-scratching conclusions: 1) We are at the center of the universe; 2) Galaxies have preposterous mechanisms that propel them through space; and 3) The universe conspires to make galaxies twice as far away from us move exactly twice as fast.
That seems like a bit of a stretch, so astronomers long ago reached a much more simple conclusion, one powered by the newfangled general theory of relativity: the space itself between galaxies is expanding, and galaxies are just along for the ride. Going big
Edwin Hubble established the expansion of the universe by cataloging nearby galaxies (after discovering that there is such a thing as "nearby galaxies"). But the story of dark energy doesn't get told by neighborhood redshifts. The game of cosmology in the latter half of the 20th century was to go deep. Way deep, which is challenging because deep-space objects are a little dim.
Thankfully, nature gave scientists a break (for once). A certain sub-sub-subclass of supernova explosions, known as Type 1a, has two useful characteristics. Because Type 1a supernovae tend to happen from roughly the same scenario — a white dwarf accretes gas from an orbiting companion until a critical threshold is reached, a nuclear chain reaction goes haywire and boom — they have roughly the same absolute brightness.
By comparing the observed brightness of a Type 1a supernova to the known true brightness (calibrated using handy nearby sources), a little high-school trigonometry reveals a distance.
But wait, there's more! Since Type 1a supernovae contain the same mix of elements, we can easily identify their fingerprint frequencies and measure the redshift, and hence a speed.
Distance and speed all in one measurement. How convenient.
Type 1a supernovae are relatively rare — only a small handful will light up each galaxy every century. But since there are so many galaxies in the universe, they're constantly popping off somewhere. And they're insanely bright, too. For a few weeks, a single explosion can outshine its entire host galaxy. That's hundreds of billions of stars for those of you keeping track.
As the light travels to our telescopes from a distant supernova, the expansion of the universe will stretch it out to longer wavelengths. The further in the past the supernova exploded, the longer the light has traveled to reach us, and the more stretching it has accumulated.
So a single supernova redshift measurement gives us the total amount of universal stretch in the intervening billions of years between us and the explosion. By performing multiple measurements at multiple distances, we can build a cosmic growth chart, mapping the expansion of the universe as a function of its age.
And that's where dark energy enters the fray. Going dark
In the 1990s, after a decade of technology development, the stage was finally set for supernovae to shed some light on the expansion of the universe. Specifically, its deceleration. In a universe full of matter, the expansion should slowly be wearing out as its gravitational pull tugs back. We didn't know how much matter was in the universe, but a measurement of the cosmic growth chart would help pin it down.
At first the results were promising: two competing groups both provided initial results of a detectable deceleration rate, but with necessarily large error bars (they were just getting started, after all). But in the coming months, things started to go downhill.
As more supernovae data came back from the surveys, the measured deceleration shrank. Then vanished. Then reversed.
It appeared that the expansion of the universe was accelerating.
Both groups frantically tried to figure out the bugs in their data-analysis pipelines. Surely something was amiss, and each was worried that the other group might steal its thunder by publishing a sound measurement while it was still fiddling with its codes.
But the data refused to budge. Nervously, cautiously, the groups reached out to each other: "Do you see what we see?"
It was then that the groups began to appreciate what the universe was telling them. Two competing teams, using different telescopes, different datasets and different methodologies, were independently coming to the same conclusion. Our universe wasn't slowing down, but speeding up.
They published their work almost 20 years ago. In the meantime, after several independent lines of evidence all pointed to the same conclusion, they shared in a Nobel Prize for their unexpected discovery.
The name for that observed phenomenon — dark energy — sticks with us today, but we still don't understand it. We don't know why the expansion of the universe is accelerating, but we do know that it does accelerate.
This is just me asking, but if the universe were expanding wouldn’t the twelve Constellations get all out of whack instead of remaining at the very same configurations as they were 2 or 3 millennia ago?
It’s even simpler than that...Since there can not be nothing, there is always something and it may be dark if you can’t see or measure it. There is always something there.
The constellations do change over time. But not due to universal expansion, but rather because, like everything in the the universe, the stars are in motion. We can’t notice any changes during the course of our lifetimes, as it takes thousands of years to become noticeable.
No.
The stars in the constellations are in our galaxy, practically next door. And given time they will in fact move from their present positions and scramble the constellations. A couple thousand years is a link of the cosmic eye....
Universal expansion occurs on the grandest of scales. ie, even structures as enormous as galaxies, with hundreds of billions of stars contained in them, basically aren’t affected by cosmic expansion. Gravity wins out at even that scale and holds them together. UE takes place in the incomprehensibly large regions between galaxies.
The DNC is dark energy
Galaxies are mostly moving apart, except our huge neighbor Andromeda which is on a collision course with our Milky Way galaxy. The collision is about 3 billion years away.
The constellations we can see with our bare eyes are all nearby stars in the Milky Way and are orbiting the massive black hole in the center of the galaxy so they are moving along with us. The stars in the Milky Way are not moving apart, they are slowly being sucked into the black hole. Generally it is other galaxies that are moving away from us (except for Andromeda).
Certain Rap Performances have a lot of energy
#DarkEnergyMatters (except that it’s energy, so it can’t matter)
Aren’t they kind of figuring out that every galaxy has at least one black hole?
Oh yeah, a space thread: Klingons! Uranus!
We can’t actually “see” any stars, except our sun. What we see are point sources of photons emitted by other suns. If it weren’t for the emitted photons the stars would be undetectable.
If they were objects illuminated by reflected light only, we could never detect or resolve the very tiny angular diameters of suns hundreds or thousands of light years distant.
All this is to establish that while all stars are in motion, the distances involved are so staggeringly vast that it takes thousands of years for them to change relative positions in our sky to a detectable extent.
But matter and energy are manifestations of the same thing. One can be converted into the other, via Einstein's famous e=mc^2.
Someone outta shine on a light on dark energy.
I beat you by one billion nano-seconds!
He’s writing for a general audience. The statement isn’t supposed to be taken literally.
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