I’ve heard it explained that the universe isn’t expanding in the sense that one volume is growing within another volume. We’re not growing into something. But rather, there’s more space being generated between things, making distances between things increase.
The analogy I’ve heard is baking raisin bread - as the loaf rises, from the perspective of any individual raisin all other raisins are getting further away from it, because the space between them is expanding.
I’ve heard the number 72 KM per megaparsec associated with this as an expansion rate.
I get the impression that this is something we’ve thought for decades at least. I think Hubble first discovered this with doppler shift observation, but I may be wrong on that.
Is this accurate?
So what, then, is dark energy? That’s something we’ve only known about for the last decade or so. It’s the concept we use to explain the fact that things are moving apart at increasing speeds - the further two things are apart from each other, the faster they get further away. Everything not gravitationally bound is getting further away from everything else.
It seems like the first thing I mentioned explains what’s going on - since space is being created everywhere, the further two things are apart the more space is created between them, and hence the faster they get away from each other. So why is “dark energy” needed as a force when cosmic inflation seems to explain it?
Or are these two different ways of describing the same thing? If so, why are the “discoveries” decades apart?
Correction: 72 km per second per megaparsec. That is to say, something 1 megaparsec away from us is getting further away from us at a rate of 72 km/s. Something 2 megaparsecs away from us is getting further away from us at a rate of 144 km/s, and so on. This quantity is known as Hubble’s constant (or more correctly, Hubble’s parameter, since it changes with time).
You’re mixing two different concepts, here. You don’t need dark energy to explain why things that are further away are receding faster. What you do need it to explain is the observation that the expansion is getting faster with time. If all gravitational effects were negligible, and I measured the rate at which a particular galaxy were receding away from us, and then measured that same galaxy again a billion years later, then that galaxy should be receding at the same speed. If there were normal, ordinary gravitational effects like the ones we’re familiar with, then when I measured that galaxy a billion years later, it’d be receding slower, or possibly even falling back towards us. But what we actually observe (indirectly, since we don’t have billions of years to make observations in) is that when we look at the same galaxy later, it’s moving away faster.
A relatively minor nitpick: In that last sentence, you mean to say “cosmic expansion”, not “cosmic inflation”. Inflation refers to a period very early in the Universe’s history when the expansion proceeded at an insanely fast rate. That period is long over: We’re still expanding, but much more slowly.
To be clear - this is in addition to any other velocities we have relative to each other, right? IOW, if the galaxies happened to be moving apart anyway based on their velocities, we’d be seperating by greater than simply the rate imparted by Hubble’s constant?
This doesn’t seem correct to me. Sure, just looking at it with Newtonian physics, if you discount gravity, we’d be moving away from each other at the same rate indefinitely.
But the rate at which we’re seperating is getting faster because more space is created between us, right? The further away we get, the more space is being created between us, and the faster we seperate. The further we get from something, the faster the rate of seperation becomes.
It seems to me that Hubble’s constant explains everything moving apart at a faster rate with no need for dark energy.
Edit: Unless it’s simply a matter that the rate which we’re moving away from each other is increasing more than even Hubble’s constant would dictate.
Yes. There can be other incidental velocities on top of that. For objects which are extremely far away, these incidental velocities are generally negligible compared to the huge cosmological effects, but for nearby objects, the incidental velocities dominate. For instance, the Andromeda galaxy, the closest major galaxy to our own, is actually moving towards us.
It would, if Hubble’s constant were a true constant. And in fact, in a universe where all of the energy density is in dark energy, it does turn out to be a constant. But there’s no a priori reason to expect it to be a constant: Hubble’s law is a description of an observed phenomenon, not an inherent reason for anything.
(Underscore mine) May I ask a question about this?
If I am looking at a galaxy 2 billion light years away, then the doppler shift I see (indicating the expansion of the universe) was what is was 2 billion years ago, right? Looking at a galaxy half the distance shows me half the expansion rate, right? Why doesn’t that mean that the exapnsion of the universe is slowing down, instead of speeding up?
Note that the Hubble constant (parameter) is not the expansion rate - the units are wrong. The Hubble constant (parameter) tells you what the expansion rate is at some distance away. If Hubble’s constant were truly constant, then as you say, the farther a galaxy is from us, the faster it moves away. General Relativity allows many solutions with a big bang initial condition. If there is enough matter, a universe can expand for a while, slowing down, and then contract for a Big Crunch. If there isn’t enough matter, then the expansion slows down due to gravity, but continues indefinitely. There is a third possibility that is the boundary between those two possibilities: the universe eventually stops expanding, but takes an infinitely long time to do so. All three cases in fact show the expansion rate slowing down with decreasing distance, or equivalently, slowing down with time, as you noticed.
Dark energy is required to “explain” why the rate of expansion is not slowing down as much as it would given the amount of observed matter, or even any amount of conserved matter. Really, it is a holding place in the equations for something we know very little about, which is why I put it in quotes.
Ok - I get that - but if Hubble observed this effect decades ago, what exactly prompted the “discovery” of dark energy in the 90s?
Let’s say two masses are a megaparsec apart with no incidental velocities, ignoring gravity for this example.
They’re moving away from each other at 72KM/sec to start, with the rate they’re moving away from each other slightly increasing over time. Eventually, they’re two megaparsecs apart - now they’re moving away from each other at a rate of 144 KM/s because twice as much distance between them means twice as much space is created between them, right?
Now Hubble seems to have figured this out - that the further away two things are, the faster they’re moving away from each other. It’s an inherent property of the way the universe is expanding, right?
Here’s where I get confused - what’s dark energy in this context? Dark energy seems to be our placeholder for a force we can’t explain which causes objects far away from each other to get even further at a faster rate. But things moving away from each other faster as they get further apart makes sense just in the context of cosmic expansion. What, then, is dark energy apart from that?
Are these two ways of describing the same thing? Dark energy is the placeholder force behind the property of the universe that Hubble is trying to describe? Why, then, were Hubble’s discoveries and the discovery of dark energy decades apart?
What new evidence did we discover in the 90s that made us think that dark energy was something we observed?
So far, so good. Hubble observed this about the time that GR came out. IIRC, that is what caused Einstein to drop the cosmological constant.
Using GR, an estimate of the amount of mass and energy in the universe, and some other reasonable approximations, it is possible to calculate the Hubble parameter. (Dark matter can be taken into account. Dark matter was added to explain the otherwise anomalous rotational behavior of galaxies. It is “dark” because it interacts very weakly, if at all, with “normal” matter and energy, except through gravity. It is also fairly mysterious, another use of “dark”, but is presumably just another kind of matter. Because its gravitational behavior is known, you can throw it in the GR equations and calculate the Hubble parameter.
The measured values of the Hubble parameter do not agree with those GR equations. To match the observed values, one supposes there is a form of energy helping the universe to expand. This mystery energy is called Dark Energy, and we know even less about it than dark matter. So far, it really is a placeholder for our ignorance. We just know that if there is a form of energy with the property that it increases the expansion rate. It could be that GR requires modifications. It could be that these are two different ways of saying the same thing. All we know is that GR, and all the forms of matter or energy that we know something about, can not explain the observed expansion rate as a function of distance of the universe.
I hope I’ve explained that well enough now. What was observed were deviations from the calculated values of the Hubble parameter as predicted by GR and our understanding of matter and energy.
What you actually do is, you measure the distance and redshift of a great many objects, and plot them on a graph. If you only measure things within, say, a billion lightyears, the graph will be more or less a straight line:
The slope of this line is Hubble’s constant. It’s hard to measure distances at cosmological scales, so all Edwin Hubble had was the low end of this graph. But if you extend that line out to greater and greater distances, it won’t stay straight: All of the prevailing theories up until the mid-1990s predicted that the line should curve downwards at least somewhat, and the big debate was just how much it was going to curve downwards. Then, though, the results came in from the Hubble Space Telescope’s capstone project, which consisted of measuring these distances and redshifts for very distant supernovae, and it turns out that not only does the graph not curve downwards, it curves up. Dark energy is what’s causing it to do that, and we didn’t discover it earlier because we didn’t have good measurements out to a great enough distance to see the graph curving.
Actually, this isn’t the best measurement of the dark energy-- That comes from measurements of the cosmic microwave background. But it is the most straightforward to explain, and was the first real evidence that came in.
This would be true if the rate of expansion were constant. However, if the rate of expansion is accelerating they will be moving faster … say 147 KM/s. “Dark energy” is the placeholder term that accounts for the additional 3 KM/s.
Just to add, the techniques for measuring distances to distant galaxies really advanced in the past two decades. When I was taking my first college astronomy classes (early 90s), Hubble’s Constant was “50~100 km/s/Mpc”. And it was based on measurements of nearby galaxies; there was no way to measure it to distances where the dark energy becomes evident. On this page about halfway down, there’s a pair of graphs that illustrate this point.
This one totally explains it for me (not that the other ones didn’t give me useful info, but this most directly and understandably answers my question) - can someone confirm that this is the correct explanation?