[ExI] Dark energy = (anti)gravity?

Sun Oct 15 22:06:37 UTC 2017

On Sat, Oct 14, 2017 Stuart LaForge <avant at sollegro.com> wrote:

> 
> Thermodynamics says that entropy is a function of state. That means it is
> path independent so it doesn't matter how the universe got into its
> initial state of zero entropy because all paths, including reversible
> ones, are equally valid.

All paths leading up to the first instant of time are equally invalid
because there were none. 

> >
>> > 
>> they don't agree,
>>  
>> some observers will say the universe is older than
>>  
>> others.
>
>
> >  
> If that is the case, then of what use is the cosmological principle?

You tell me. T
he cosmological principle
 says the universe looks the same at the largest scale, but it doesn't.
Very distant galaxies near the limit of our cosmological horizon are
smaller but have larger stars in them than galaxies that are closer to us.

> 
> Why
> 
> even bother talking about the universe as some distinct entity unto itself
> that can have a definable age?
>

Nothing has an age everybody can agree on, but that doesn't prevent
anybody talking about stuff.

>
>
>> >
>>  If a traveling
>> 
>>  observer goes from point A to point B the Proper Time of
>> t
>> hat journey is the time measured by the observers own stopwatch and using
>> 
>>  the traveling observer's definition of simultaneity to decide when to
>> start and stop the stopwatch. But there is no universal agreement, some
>> 
>> observers will say the stopwatch is running too slow, others will say it
>> 
>> is too fast, and they will say the traveling observer started and stopped
>> 
>> the watch at the wrong time.
>
>
> >
> But in the case of the universe, all those stop watches started at the
> same time and in the same place.

But to measure a time interval both a start and stop point is needed and all
those stopwatches stopped at different times because there is no universal
agreement on simultaneity, so there is no agreement on if watch X stopped
before watch Y or watch Y stopped before watch X. And to make things even
worse the stopwatches are running at different rates. So there is no
universal agreement on when the Big Bang happened; right here right now we
say it was 13.8 billion years ago, but others would disagree

> > 
> They would have gotten out of synch over
> 
> the years due to local space-time curvatures but you should be able to
> 
> average all those stop watches together and get something like a "true
> 
> age" of the universe.
>

There are a infinite number of ways a bunch of distant clocks can be
brought together in a expanding universe, and I don't see how everyone
could agree on how to do it. Should distant clock X be brought to clock Y
or should clock Y be brought to clock X? It makes a difference because one
clock would be accelerated and the other clock would not and a accelerated
clock runs slower than one in a inertial frame of reference.

> >
>  a flat universe can
> 
> conserve energy and that is thermodynamically satisfying.

A universe can be flat and still be
expanding and even 
accelerating if there is a property of space itself that causes it to
intrinsically 
contain energy, and we found out 20 years ago that there is, about 1/100 of
a
n
erg per cubic meter. A erg is about as much energy as a common housefly
needs to perform one push-up so that may not sound like much but there is a
awful lot of otherwise empty cubic meters out there, so much so that today
dark energy makes up 70% of the mass/energy in the entire universe. And
that percentage will increase as time passes because both normal matter and
dark matter will keep getting diluted but dark energy will not, the more
space
that space itself creates
the more dark energy there is
,
but the amount of matter
in the universe 
will be constant.

> 
> And the philosophical benefits of an infinite universe are also
>  
> satisfying. It would mean that we too are infinite with countless copies
>  
> repeated through time and space across the cosmos. Countless versions of
>  
> us living identical lives. Countless versions of us living every possible
>  
> permutation of our lives. Infinite copies of us taking every possible road
>  
> almost all of which are unique.
>

I find that philosophically satisfying too, and maybe its true but the
universe is under no obligation to conform to human desires.  

> > ?From  Sean Carroll at:
> > http://www.preposterousuniverse.com/blog/2010/02/22/energy-
>
> > 
> Here Carroll makes it clear that he denies the conservation of energy to
> avoid having to explain negative energy to people. It's a pedagological
> choice he makes, not one based on mathematical reasoning.

The only reason humans invented conservation laws is to help us understand
how the world works. If in circumstances far from everyday life we have to
contort them in complex ways so they still apply then there gets to be a
point where it's not worth the effort. The important thing is we
can us the

mathematical reasoning

in Noether's theorem
 to conclude that if the fundamental laws that tell objects how to move
do not change with the passage of time then energy is conceived, but
General Relativity says they do
change
with the passage of time. I go back to my example of a gamma ray photon
produced in the Big Bang, because something very fundamental has changes
since that photon was produced, space has expanded, that gamma ray photon
is now a far less
energetic microwave photon and eventually space will have expanded so much
it will be a radio photon with a wavelength

longer than the observable universe and be undetectable even in principle.
The energy in that photon would have been conserved if space didn't expand,
but it does so it isn't.

>
> > 
> keep in mind that in a flat universe Dc is not just the critical
>  
> density of the universe but also the actual density of the universe.
> 
>

The equation you're using, Dc = 3H^2/(8*pi*G)

,
where Dc is the critical density 
is only valid if the cosmological constant is zero, but we've known for 20
years that i
t
can't be zero because the universe is accelerating. So density alone
doesn't determine geometry of the universe and
thus we can't be living
in a
 simple

Friedmann universe
.

> > 
> And H is only decreasing with time only if the density of the universe is
>  
> likewise decreasing with time.
>  
> But if the density of the universe is
>  i
> ncreasing with time through conservation of energy

The density of normal matter and Dark Matter has decreased over time but
it has become clear that the gravitation caused by matter alone (not even
with the help of dark Matter) is insufficient to explain the evolution of
the universe. For a very long time the expansion of the universe was
slowing down just as you'd expect, but about 5 billion years ago (and
nearly 9 billion year after the Big Bang) the deceleration stopped and
things started to accelerate. This can only be because the matter became
diluted and so did the gravitational force trying to slow things down but
some property of space itself called Dark Energy which nobody understands
causes things to speed up, so whatever it is when there is more space there
is more Dark Energy   

> >> 
>> communicating is not the same thing as influencing, communicating
>> 
>>  involves transferring Shannon style information and  entanglement can't
>> do that faster than light. But it will still let you influence
>> 
>>  things faster than light.
>
>
> > 
> Good. That's all my theory needs is for gravity to be able to "influence
> things" faster than light. No Shannon entropy need be exchanged.
>

But you also said:

*"*
*In a flat universe, dark energy is just superluminal gravity at long
ranges*
"

I'm not sure what that means. If Dark Energy is a property of space itself
as if seems to be then it doesn't need to travel a long distance to be
manifest. 

> 
> My earlier attempts at quantum gravity have been overturned by the
> super-long Compton wavelength of the graviton reported by LIGO.

I don't know what you mean by that, LIGO has not detected the graviton nor
has anybody else, it is purely theoretical and may not even exist and even
if it does I think its unlikely anyone will be able to find one this
century. 

> 
> My equations simply lump pressure and tension together with matter density
> and radiation density through the mass-energy equivalence principle. It
> just deals with total density of all components of the stress-energy
> tensor converted to mass.
>

Both pressure and tension are potential energy, but Einstein says pressure
causes gravitational attraction but  tension (negative pressure) causes
gravitational repulsion.

  John K Clark
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