[Hannah LS (OP)]

Michael asks:

When I hear that the universe is expanding faster and faster, I question whether this is just because the first pieces leaving an explosion go a lot faster than the later pieces. This would mean that the further out you look the faster the expansion appears. Could this be the reason?

What do you think?

[Kryptid]

The Big Bang wasn't an actual explosion, it was a rapid expansion of space itself. It's an unfortunate, yet widespread, misconception to think of the Big Bang as an explosion akin to a giant bomb.

[yor_on]

You sit in your chair looking out on the stars in a telescope. You have a devise that can measure the lights 'energy'/frequency' and it tells you that the longer you sit there looking the 'colder' the light becomes. It seems to lose energy as time pass you by, from the same star? How would you explain it? One way is to think of a ambulance passing you in the street, as it comes up to you it sounds a lot shriller than once it passed you, disappearing.

The 'waves' of its siren became compressed relative your ears hearing as it was coming up to you, but as soon it was moving away from you those 'waves' took a longer time reaching you, and so the sound of the siren becomes 'red shifted' relative your space time position.

That's how cosmological red shift comes to be, and it is then presumed that this is a proof of the universe expanding.

The Big Bang is you sitting in the chair measuring the time it takes for the furthest stars light to reach you. 13.7~ lightyears away. Assume you could teleport instantly to another chair one billion ly years away, and do the same measurement from there. Would you expect to see the light there from the farthest star to be 12.7~ light years now (you looking in a same direction)? If you would, that would be equivalent to placing Earth as the 'center of the universe' :)

No.

Not possible. First I need to believe in unicorns to do that.

We can make it even more fun if you then would look back to Earth, because the furthest star would then need to be located 14.7~ ly away.  And If I would want to be utterly correct I would then need to define the Big Bang to have started at the mass center of my chair, where I was first, sort of :)

As for being possible, not really.
=

The other side is that the Big Bang always start at your chair, no matter 'where or at what time' that chair is :)
Is that more possible?

Yes, because then it will be a question of how we define a universe.

[Bill S]

The expansion “mechanism” raises some interesting thoughts.

Spacetime is “something”; something is expanding. 

(1) If the Universe is homogeneous, and in order to do any meaningful cosmology we must assume it is, we cannot define a physical boundary

(2) We cannot define any part of the Universe; rather than any other part; as being the newly created part. 

This must mean that every part of the Universe is expanding; new spacetime is coming into existence everywhere in the Universe, all the time, or the “density” of spacetime is decreasing. 

Can spacetime be more or less “dense”?  If so, what effect does stretching space and time have at a practical level?  Undoubtedly the simplest answer would be that new space and time are continuously being created between and within the galaxies so that spacetime everywhere remains constant in its “density”.  I think this seems to be accepted, albeit cautiously, by many scientists.

[jeffreyH]

I wouldn't say that new time is created since that makes no sense. New space is created by the expansion. There will be an effect upon time dilation since the gravitating bodies that affect it are being pushed further apart. This effect is hardly detectable though. Since the effects of gravity will be minimal anyway in deep space. This is where the force of dark enery takes over.

The interesting stuff happens when gravitational effects become very strong.

[Bill S]

I wouldn't say that new time is created since that makes no sense. New space is created by the expansion.

 

I would agree with that, but Minkowski assures us that time and space are inextricably interwoven, and that neither can exist as an individual entity. 

Be that as it may, it also seems reasonable to regard spacetime as a concept that is valuable, but, perhaps, should not be taken too literally.  Space expands, so we have to adjust our measurement, and time is what we use to achieve that measurement. 

[jeffreyH]

Time and space are linked via relativity and depend upon Lorentz transformations. That still doesn't mean that you can create new time. Time is not a dimension of distance. It can be used in line element calculations which equate to distances. Spacetime is a mathematical concept. No more or less should be made of it.

[jeffreyH]

One thing to make note of is that every point in space will have a unique vector pointing through it. This vector will be the result of summing all the force vectors that point through it from every direction. Normally this will be undetectable. At other times, in the case of gravitation, these will all cancel to a point. These are Lagrange points. All fields must be included. This means that a dark energy field must also contribute. This could be one reason why galaxies and the interconnecting structures arose the way they did after the big bang.

[Bill S]

Spacetime is a mathematical concept. No more or less should be made of it.

That’s certainly not something I would argue with, even were I qualified to do so.

Returning to the expansion process; new space is being created everywhere in the Universe, all the time; but space is not empty. 

Interstellar space has an average density of one atom per cm³.  Then there are all those virtual particles; so are new atoms and virtual particles being created, or is the average density of the Universe decreasing? 

[guest47899]

view the cosmic radiation background as a template. project the colder blue regions outward as a model for the cosmic filament web network. project the warmer yellow/red regions also outward past the filament web network, this represents the outer boundary of space/time. The filament web network rotates 360 degrees, as does the warmer microwave boundary at the edge of space. when the microwave boundary interacts with the remnants  of the recombination periods quantum fluctuations at the edge of space, new space/time is created. The space/time we inhabit is cold blue microwave radiation. as new space/time is created, the filament expands proportionally.

The above may also imply that the cmb is merely a inner spherical 360 degree reflection of itself, expanding outwardly.


 

[jeffreyH]

Spacetime is a mathematical concept. No more or less should be made of it.

That’s certainly not something I would argue with, even were I qualified to do so.

Returning to the expansion process; new space is being created everywhere in the Universe, all the time; but space is not empty. 

Interstellar space has an average density of one atom per cm³.  Then there are all those virtual particles; so are new atoms and virtual particles being created, or is the average density of the Universe decreasing? 

I would expect the density of particles to drop. The virtual particles are temporary disturbances that arise from the vacuum so I would say that there are the same 'density' as before. Although density is a bad choice of word.

[jeffreyH]

BTW This relates to the energy of the vacuum and the so called vacuum catastrophe.
https://en.m.wikipedia.org/wiki/Cosmological_constant_problem

[Bill S]

This means that a dark energy field must also contribute.

Would I be right in thinking that the acceleration of the expansion of the Universe is thought to be caused by dark energy that fills space at a density of 10^-10 joules per cubic metre?

As space expands, this energy density must not decrease, or the acceleration would lessen.  (?)

This suggests that either the concept of dark energy violates a basic principle of physics, or energy is not necessarily conserved in GR.

Would this mean that time-translation invariance is negated by GR?

[jeffreyH]

This means that a dark energy field must also contribute.

Would I be right in thinking that the acceleration of the expansion of the Universe is thought to be caused by dark energy that fills space at a density of 10^-10 joules per cubic metre?

As space expands, this energy density must not decrease, or the acceleration would lessen.  (?)

Since the expansion of the universe is accelerating it all depends upon whether or not this acceleration is linear or not. A non linear acceleration would indicate an increase in dark energy. I haven't studied this in any depth so I can't state an opinion either way.

This suggests that either the concept of dark energy violates a basic principle of physics, or energy is not necessarily conserved in GR.

Would this mean that time-translation invariance is negated by GR?

I don't know enough to comment on your last two points. Maybe others can chip in.

[Bill S]

Since the expansion of the universe is accelerating it all depends upon whether or not this acceleration is linear or not. A non linear acceleration would indicate an increase in dark energy.

Presumably, a linear acceleration would just require the density of dark energy to remain constant.

[jeffreyH]

Since the expansion of the universe is accelerating it all depends upon whether or not this acceleration is linear or not. A non linear acceleration would indicate an increase in dark energy.

Presumably, a linear acceleration would just require the density of dark energy to remain constant.

Yes.

[DouglasH]

With the help of modern science and communication universe is getting small day by day. What do you think?

[Bill S]

With the help of modern science and communication universe is getting small day by day. What do you think?

I think that's a matter of perception, rather than physical reality.  When I was 8, a year was a long time; now that I'm nearing 80, it's too bl**dy short.

[yor_on]

Cool Bill, you're a proof of that the mind never stops working.
I'll try that too :)