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.
There are three known types: Doppler shifts ( due to motion through space away from the observer); gravitational redshifts ( due to light leaving a strong gravitational field); and cosmological expansion ( where space itself stretches as light travels through it). The way astronomers distinguish between the three depends on the kind of object they are studying. Here's a table of the different kinds of objects and the liklyhood that one of these three is present to create the observed spectral shift.
| Object | Doppler | Gravitational | Cosmological |
| Planets | x | x | |
| Stars | x | ||
| Nebulae | x | ||
| Neutron Stars | x | x | |
| White Dwarfs | x | x | |
| Nearby Galaxies | x | x | |
| Distant Galaxies | x | x | |
| Black Holes | x | x |
Gravitational red shifts are generally very small, and you only get very large ones from the light emitted near neutron stars or black holes...environments you can independently confirm from other observations. Cosmological redshifts are only important and easily distinguishable for rather distant galaxies, but can get mixed up with the Doppler shift from the regular spatial motions of galaxies. With the exception of the sun, no gravitational red shifts have been detected for ordinary stars, but they ought to be present if we had good enough instruments.
Mainly, to distinguish gravitational redshifts from other kinds, you compare the size of the object with its mass to determine how much larger it is than its black hole radius. Objects like nebulae and entire galaxies are trillions of times larger than their BH radius, so the magnitude of the redshift is 1 part in a trillion of the rest frequency. Normal stars are only a few hundred thousand times larger than their BH radius, so light from their surfaces is at the limit of being able to detect, spectroscopically, such a gravitational redshift. Neutron stars and white dwarfs are about 10, and 3000 times larger than their BH size so gravitational redshifts are of the order of 1 part in 10 to 1 part in 1000 of the rest wavelength.
Cosmological redshifts are only seen unambiguously at distances of 100s of megaparsecs. At nearer distances, ordinary Doppler shifts from galaxian motion with respect to a local center of mass ( galaxy cluster) is comparable to the cosmological effect and you have to disentangle the two contributions very carefully. Typical galaxy speeds in a cluster are 300 km/sec, and this equals the cosmological recession at a distance of only 5 megaparsecs or so!
https://einstein.stanford.edu/content/relativity/a11859.html
This doesn't make sense to me. The red shift occurs at the moment the light left the object - what is stretching it out after that time?
As the light is traveling through space, space itself is expanding, thereby stretching the lightwave. This phenomenon is independent of whether the object itself is physically moving towards us or not. Physical motion is reflected in standard Doppler shift. Further, if the light source has a strong gravitational field associated with it, there would be yet another (a 3rd) shifting of the light frequency.
As the piece I posted above notes, there are 3 (known) types of shifting: shifting due to actual motion of a light source, shifting due to a strong gravitational field, and shifting due to universal/cosmic expansion. Cosmic expansion and gravitational shifting is always redshifted, whereas standard Doppler involves an object physically moving EITHER towards OR away from us.
For example, light from a remote high mass/high gravity star which, say, happens to be physically moving towards us, would be shifted in all 3 ways.
There would be blueshifting due to the fact that it (the star) is physically (actually) moving towards us, And separate redshifting due to its strong gravitational field AND cosmic expansion. There actually are ways to sort out which effect is causing each shift.
In my example above, instead of star, I should have said galaxy, as it would be impossible, or nearly impossible, to detect such subtle shifting in individual stars at distances where cosmic expansion comes into play.
“You’ve experienced the Doppler effect if you’ve ever had a train go past you and heard the whistle go to a lower pitch (corresponding to a longer wavelength for the sound wave) as the train moves away. The Doppler effect can happen for light waves too (though it can’t be properly understood without knowing special relativity). It turns out that just like for sound waves, the wavelength of light emitted by an object that is moving away from you is longer when you measure it than it is when measured in the rest frame of the emitting object.
In the case of distant objects where the expansion of the universe becomes an important factor, the redshift is referred to as the “cosmological redshift” and it is due to an entirely different effect. According to general relativity, the expansion of the universe does not consist of objects actually moving away from each other - rather, the space between these objects stretches. Any light moving through that space will also be stretched, and its wavelength will increase - i.e. be redshifted.
(This is a special case of a more general phenomenon known as the “gravitational redshift” which describes how gravity’s effect on spacetime changes the wavelength of light moving through that spacetime. The classic example of the gravitational redshift has been observed on the earth; if you shine a light up to a tower and measure its wavelength when it is received as compared to its wavelength when emitted, you find that the wavelength has increased, and this is due to the fact that the gravitational field of the earth is stronger the closer you get to its surface, causing time to pass slower - or, if you like, to be “stretched” - near the surface and thereby affecting the frequency and hence the wavelength of the light.)
Practically speaking, the difference between the two (Doppler redshift and cosmological redshift) is this: in the case of a Doppler shift, the only thing that matters is the relative velocity of the emitting object when the light is emitted compared to that of the receiving object when the light is received. After the light is emitted, it doesn’t matter what happens to the emitting object - it won’t affect the wavelength of the light that is received.
In the case of the cosmological redshift, however, the emitting object is expanding along with the rest of the universe, and if the rate of expansion changes between the time the light is emitted and the time it is received, that will affect the received wavelength.
Basically, the cosmological redshift is a measure of the total “stretching” that the universe has undergone between the time the light was emitted and the time it was received.”
Point of information: There are 88 constellations, 12 of which on the ecliptic (the projection of earth's orbital plane onto the sky) make up the Zodiac. The Zodiac is important to astrology because the orbits of the other planets are confined to stay within it. The ancient Babylonians charted the position of the sun against the Zodiac by noting the position of the moon among the stars during a lunar eclipse, and correctly assigning the sun the opposite position.
Your zodiacal sign roughly corresponds to the position of the sun along the zodiac on the calendar date of your birth at the time that the Julian calendar was adopted. The dates of the location of the actual sun have changed by about a month earlier due to precession of the equinoxes, a phenomenon apparently unrecognized by the Babylonians, but completely familiar to Greek astronomers.
E pur si muove.
Gravitational shifting can be towards the blue, as when a photon approaches a black hole. Or the re-tuning of the atomic clocks on GPS satellites.
When things are thousand/millions of light years away, a lot of motion is next to imperceptible over a few thousand year time-frame.
The AG is estimated to be approximately 2.3 million light years away -- one light year being the *distance* light travels in a year at its constant speed of 186,000 miles per second. It works out to about 5.9 trillion(with a T) miles.
They believe Andromeda will "mingle" with our Milky Way galaxy in about 4 or 5 billion years. That's about the same time when our Sun should be in the process of destroying itself by moving to the Red Giant stage. This will happen when the Sun uses up much of the material it needs to continue nuclear processes. Without that outward push from nuclear reactions gravity will win out and cause an implosion. The implosion, in turn, will ultimately result in a "rebounding" which will cause the Sun to grow to an enormous size and eventually engulf all planets up to Mars.
Is that in any way detectable?
Or the re-tuning of the atomic clocks on GPS satellites.
I know that GPS technology takes into account Relativity theory, both Special and General. Special deals with the high rate of speed of the satellites and the effects that has on time (between the sat and ground), while General deals with the effects of time due to Earth's gravitational field, which of course is stronger at the surface than high above it at the sat's altitude.
Interestingly, as the speed aspect causes the sat's clock to tick more slowly (from the ground's perspective), the increased gravity at the surface causes the ground-based clock to tick more slowly. The surprising (to me anyway) net result is that the gravitational effects win out slightly over the speed effects, and so the earth-bound clock ticks out time more slowly than the sat's, and therefore adjustments need to be made to insure better precision of the system.
Red Giant Phase:
In 5 billion years from now, the Sun will enter what is known as the Red Giant phase of its evolution. This will begin once all hydrogen is exhausted in the core and the inert helium ash that has built up there becomes unstable and collapses under its own weight. This will cause the core to heat up and get denser, causing the Sun to grow in size.
It is calculated that the expanding Sun will grow large enough to encompass the orbit’s of Mercury, Venus, and maybe even Earth. Even if the Earth were to survive being consumed, its new proximity to the the intense heat of this red sun would scorch our planet and make it completely impossible for life to survive. However, astronomers have noted that as the Sun expands, the orbit of the planet’s is likely to change as well.
When the Sun reaches this late stage in its stellar evolution, it will lose a tremendous amount of mass through powerful stellar winds. Basically, as it grows, it loses mass, causing the planets to spiral outwards. So the question is, will the expanding Sun overtake the planets spiraling outwards, or will Earth (and maybe even Venus) escape its grasp? ...”
https://www.universetoday.com/12648/will-earth-survive-when-the-sun-becomes-a-red-giant/
The received frequency of GPS satellite signals is higher than the transmitted frequency, the definition of blue shift. The bigger problem is that the same oscillators are used to create the transmitted waveforms as are used to time transmissions. The start of frames are multiples of 1.5 seconds. One could use a time scale that beat seconds at the rate of the orbiting GPS constellation and let it march ahead of terrestrial clocks, so long as the constellation was mutually synchronized. The drawback to this approach would be that “GPS time” would have no simple relationship to terrestrial time scales. A side benefit of GPS is that it affords a very simple and highly accurate method of time synchronization worldwide. GPS clocks are steered to closely track UTC for this purpose.
Thank you for the reply. I can see I am way out of date. I was okay with gravitional curving of space, but space expanding in and of itself, not just the things in the space moving apart, is going to take some effort to get my head around. Could you recommend a good book to read?
Saw this article a while back. Yes they change.
https://spaceplace.nasa.gov/review/dr-marc-space/constellations.html
Our ideas about the universe are limited to what we believe, scientifically, from what we theorize, from only what of it we can observe and limited by the primitive powers of our observation instruments.
Actual space exploration will use very little of our theories about the universe, as to its “size” and how much “matter/energy” it has - for ages to come - relying only on what empirical proven knowledge we have.
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