[ExI] Dark energy = (anti)gravity?
johnkclark at gmail.com
Sun Nov 19 03:27:18 UTC 2017
On Thu, Nov 16, 2017 at 11:32 PM, Stuart LaForge <avant at sollegro.com> wrote:
> How do you know there was a first moment of time?
I don't know that, but I do know if there was a first moment of time then
it would be silly to talk about paths leading up to it.
> If any observer can use that space-time interval to calculate what any
> other observer sees, that is in effect, universal agreement.
So if I say your clock is running
slow and you say my clock is running slow we are in agreement? Which event
do we agree happened first, event X or event Y? At the end of the day do
we both agree that I turned out to be wrong or do we both agree that you
turned out to be wrong?
> If that is not universal agreement,
> then what is?
We both agree that your clock was running slower than mine. And that is
what would happen if you revered your velocity and came back and we placed
the two clocks side by side and we both looked at them.
> But those who disagree on which
> happened first can't ever communicate with one another, though, because
> they are in different causal cells.
No, we were never moving faster than light so we can communicate again.
We're both moving apart and we both see the other person's clock running
slow, but then you turn around and come back and now we're back at the same
place and we're not moving with respect to each other anymore and we both
find that our two clocks do not agree, we both agree your clock ran slow,
mine says 3 o'clock but yours says 2 o'clock. There is no paradox because
the symmetry has been broken, you accelerated but I did not, if you're
accelerating (or equivalently in a strong gravitational field) you will see
my clock running fast and I will see your clock running slow.
> If there are always at least two observers for which energy is conserved
When looking at the largest scale no observer will ever see energy
conserved in a accelerating universe.
> there must be places where the big bang has not yet happened.
Places? The Big Bang was the origin of places, before the Big Bang there
were no places, at least not in our observable universe.
> only other alternative is that it is simultaneously happening everywhere
If the Big Bang was the beginning of time then it was simultaneous because
no other time existed when it could have happened, and it happened
everywhere because it was the beginning of space so there was no other
place it could have happened.
> in an infinite universe, big bangs are as common as you are.
Yes I agree, but if they exist those places
really are causally disconnected from us.
> But we would both agree on how much fuel you used to get here no matter
> when you hit the accelerator or the brake. Which is what we are
> discussing. The conservation of energy.
The conservation of energy still works fine at small scales, a million
light years or so, but not at cosmological distances.
>> why the universe was decelerating for the first 9 billion
>> years of its existence and only started accelerating 5 billion years ago.
> It's because the rate of expansion of space-time in our causal cell is
> linked to the rate of matter creation in our causal cell. But they balance
> each other out because
> the negative energy of that all that curved
> space-time exactly cancels out all the positive mass-energy.
But things are not balanced out and never have been, if they were the
universe would be expanding at a constant rate but instead it was slowing
down for the first 9 billion years and then sped up for the next 5 billion.
And that what you'd expect if matter, which wants to slow things down, was
becoming more dilute as more space was created but Dark Energy, which wants
to speed things up, was not becoming more dilute because it is a property
of space itself.
>> I don't insist on matter being conserved but not energy, but for the last
>> 20 years we've have lots of empirical evidence that the universe does
>> insist on exactly that. As to why the universe prefers things that way
>> you'll have to ask the universe not me.
> But the mass-energy equivalence principle makes that paradoxical. If you
> can convert matter into energy and vice-versa, then that means that matter
> cannot be conserved or energy must be conserved. You can't have it both
Water and steam can be converted from one to the other but that doesn't
mean the laws of physics treat the two things the same way. Einstein said
matter and energy can be converted from one to the other but he certainly
did not say they always are converted or that the two things
identical. The amount of matter in the universe, both dark and regular, has
remained almost constant since the Big Bang, but the amount of Dark Energy
and its increased by a lot
. The amount of matter and Dark energy has changed and that's
the expansion the expansion rate has also changed.
> If the number of particles increases, and their rest masses are conserved,
Then each individual particle must have less mass than before.
> Why is the negative energy density of dark energy i.e. negative
> pressure or tension assigned a positive mass such that it accounts for 68%
> of the universe's mass?
We know that there is about 5 times more Dark Matter than regular matter
and we know that all matter has mass or it wouldn't be matter. Energy has
no rest mass but matter does and that means it takes energy to accelerate
it, and something is accelerating all that matter and the energy required
to do that would be about twice as much as energy equivalent of all that
matter, dark and regular combined.
> when 3 protons fuse they produce 4 particles, a positron a neutrino a
> gamma ray and a Helium-3 nucleus, but the number of particles is
> irrelevant from a cosmological perspective, but the total mass/energy is
not and the fusion process does not change that.
> I am suggesting it is not irrelevant. 4 particles far apart bend spacetime
> differently than 3 particles close together.
The 4 particles bend spacetime less than the 3 particles did because there
is less mass, the gamma ray has no rest mass but it took rest mass to
create it in the fusion process.
> A flat or negatively curved infinite universe can expand and even
> accelerate indefinitely.
It can expand
but you can't deduce acceleration from geometry alone, you need energy
> If it is flat, then Freidmann demonstrated that
> Einstein's theory predicts that energy is conserved.
And Freidmann did not predict a
accelerating universe nor did anyone else, it came as a great shock to
everyone, its still not understood worth a damn and that's why its called
dark. It's been known for a long time from the laws of Quantum Mechanics
that empty space should have energy, but the calculated value was HUGE,
about 10^120 times larger than what we actually see. If it were really that
big the universe would blow itself apart a nanosecond after the Big Bang.
Physicists weren't too worried about that because they figured eventually
they'd figure out a way to cancel things out and the true value
zero. But now with the discovery of a accelerating universe they must
figure out how to cancel everything out EXCEPT for one part in 10^120, a
far more difficult task.
Dark Energy is the greatest mystery in physics, even stranger than Dark
> Dark matter is probably black holes
That would be my best guess too but I could be wrong.
> In a Dark Energy-dominated universe what's the point of having any matter
> at all?
I don't think the universe has a point, but people do.
> The number of particles *is* relevant. The gravitational field stores
It can convert kinetic energy to potential, but in fusion the kinetic
energy itself had to come from someplace and that would be the rest mass of
the original particle, so I don't see the point of considering it.
> it's outside our light-cone so we can
>> never know.
> But with gravity you can detect things you can't see.
If something is outside our light cone it can't effect us and we can't
effect it, not by by gravity and not by anything else that moves at the
speed of light or less.
And everything moves at the speed of light or less.
> It's how we detected
> the planet Neptune by how it perturbed the orbit of Uranus
Neither Neptune nor Uranus is outside our light cone. And we can see
Neptune we just didn't know where to look.
John K Clark
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