I'm a bit rusty on all this stuff. From Dark energy (wikipedia):
The "cosmological constant" is a constant term that can be added to Einstein's field equation of general relativity.
If considered as a "source term" in the field equation, it can be viewed as equivalent to the mass of empty space
(which conceptually could be either positive or negative), or "vacuum energy".
The simplest explanation for dark energy is that it is an intrinsic, fundamental energy of space.
This is the cosmological constant, usually represented by the Greek letter Λ (Lambda, hence Lambda-CDM model).
Since energy and mass are related according to the equation E = mc2, Einstein's theory of general relativity predicts
that this energy will have a gravitational effect.
It is sometimes called a vacuum energy because it is the energy density of empty space - the vacuum.
There is a minor problem with this though:
the discrepancy between theoretical calculation and observed value is 120 orders of magnitude!!! ...
making it a leading candidate for "the largest discrepancy between theory and experiment in all of science". :)
On a more positive note, the precision of narrowing down the Age of the universe
(via the Lambda-CDM model) to 13.772±0.040 billion years blows my mind.
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No, you're getting confused: vacuum energy and dark energy are not the same thing. The latter is purely theoretical, hypothesized as an explanation of the cosmological constant; it has never been observed, and we have no idea what it is - if it exists - so there's no way for there to be a discrepancy. Or maybe I'm confused. Whatever. ;-)
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> No, you're getting confused ... Or maybe I'm confused. Whatever. ;-)
Confusion and total bafflement is what makes this fun. :)
Even the experts are confused:
- If you are not completely confused by quantum mechanics, you do not understand it (John Wheeler)
- Those who are not shocked when they first come across quantum theory cannot possibly have understood it (Niels Bohr)
- I think I can safely say that nobody understands quantum mechanics (Richard Feynman)
You seem to be objecting to a direct quote from wikipedia ...
which I (gutlessly) used to reduce the risk of upsetting people with my ignorance.
Since that didn't work, I'll try answering in my own words this time. :)
> vacuum energy and dark energy are not the same thing
Agreed.
Noone knows what Dark Energy is.
To date, all attempts to observe dark energy directly in a laboratory have failed.
The simplest explanation is that Dark Energy is "the energy of empty space" aka "vacuum energy".
And "vacuum energy" and "the energy of empty space" and "the cosmological constant aka Lambda" are used interchangeably,
at least that's what I see, e.g. see Lambda-CDM model.
After the early GR Cosmological Constant fiasco, which Einstein described as "his biggest blunder"
(because it doesn't work - local random variations would still lead to either runaway expansion or contraction,
not the static universe he wanted),
everyone just assumed that the Cosmological Constant was zero and didn't worry too much about it.
Feynman and Wheeler later made a theoretical quantum field theory estimate that the zero-point energy of the vacuum
is an order of magnitude greater than nuclear energy, with a single light bulb containing enough energy to boil all the world's oceans!
(see also: the cosmological constant problem aka the vacuum catastrophe).
Though disturbing, people didn't seem too worried when the
Cosmological Constant (aka the energy of the vacuum) was zero.
For some reason, it caused much more angst after the experimental Dark Energy bombshell of 1998/1999 (independently measured by two different teams)
that it was not zero after all, but a tiny obscure positive value that makes so little sense that some
have attempted to explain its value via the Anthropic principle in a multiverse.
Further Update: To illustrate how these terms are bandied about by experts,
I'll add more quotes as I come across them in my reading.
From the book The God Equation (2021) by renowned physicist Michio Kaku, at the bottom of page 137:
What might be driving this runaway expansion is something that was once discarded by Einstein in the 1920s,
the cosmological constant, the energy of the vacuum, now called dark energy.
Surprisingly, the amount of dark energy in the universe is enormous.
More than 68.3 percent of all matter and energy in the universe is in this mysterious form.
... If one tries to blindly calculate the amount of dark energy in the universe
(using the assumption of relativity and the quantum theory), we find a value
that is 10**120 times larger than the actual value! ...
This is the largest mismatch in the entire history of science.
From the book The End of Everything (2020) by astrophysicist Katie Mack, at the bottom of page 80:
Because we don't know whether it's really a cosmological constant or not, we generally call any hypothesized phenomenon
that could make the universe accelerate in its expansion dark energy.
To throw some more terminology into the mix, an evolving (i.e. nonconstant) dark energy is often called quintessence,
after the "fifth element", a mysterious something-or-other that was popular to philosophize about in the Middle Ages
and is not really much more precisely specified now ... we know that whatever it was that caused cosmic inflation
eventually turned off, so perhaps a similar accelerated-expansion-causing field could have turned on since then,
causing the acceleration we see today ...
In any case, based on observations at the moment, it really looks a lot like dark energy is a cosmological constant:
an unchanging property of spacetime that has only recently (i.e. in the last few billion years) come to dominate
the evolution of the universe.
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"explanations" are in physics nowadays little more than temporary metaphors, helping to somehow memorize equations.
Relevant is that (competing) models predict as many observations/experiments as possible.
And this happens with equations in mostly mathematical models.
The way how these formulas are interpreted (I'd rather say "motivated") is disputed and changing quite frequently (and often even not possible).
Mathematical instruments can evolve (non-euclidean geometry? string theory?), metaphors ("indivisible" Atom?) can change, CGI artists working for Sagan and his successors can invent new imaginations.
At the end most people only remember things like an animated reddish Einstein approaching light speed on a bike in a genius-pop-culture.
Like having weird hair and showing your tongue or sitting in a wheelchair with a voice computer are attributes of a brainy superhero.
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