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But, as per the Law of Conservation of Matter, matter can neither be CREATED, nor be destroyed...

So, shall we conclude by saying that Universe existed earlier also and would tend to exist forever?

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- Thread starter Priyank
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But, as per the Law of Conservation of Matter, matter can neither be CREATED, nor be destroyed...

So, shall we conclude by saying that Universe existed earlier also and would tend to exist forever?

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Bandersnatch

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1. BB says nothing about the creation process. Its applicability stops short of the singularity that you end up with if you try and extrapolate the model too far back in time.

2. It's impossible to clearly define energy on the cosmological scales. If you can't even say what energy is, you can't say anything about its conservation or lack thereof.

Here's some further reading on these two points:

Re 1 - http://www.einstein-online.info/spotlights/big_bangs

Re 2 - http://www.preposterousuniverse.com/blog/2010/02/22/energy-is-not-conserved/ and http://math.ucr.edu/home/baez/physics/Relativity/GR/energy_gr.html

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In addition to what Bandersnatch said, be advised that this is a complete mis-statement of the law, which is actually called the Law of Conservation of Mass and says that MASS cannot be created or destroyed in a closed system. Matter gets destroyed all the time but the destruction results in no loss of mass since the radiation portion has energy which is equivlent to mass in this context.... as per the Law of Conservation of Matter, matter can neither be CREATED, nor be destroyed...

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ChrisVer

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marcus

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Sure....So, shall we conclude by saying that Universe existed earlier also and would tend to exist forever?

Why shouldn't we take that as a working hypothesis and try to improve the cosmology model so it does not break down at the very start of expansion?

Why not take an improved model that does not fail and develop infinities, try to push back in time to a little bit BEFORE the start of expansion? Let's see what we find.

People are doing that. A good recent paper is the December one by Edward Wilson-Ewing and Yi-Fu Cai. It's comparatively simple and straightforward, but it is not popularization, it's professional research journal type. the title is "LambdaCDM Bounce Scenario"

You get a link to the "arXiv.org" PDF text to download or read online, if you google "LambdaCDM bounce"

LambdaCDM is the name of the standard cosmic model that essentially all cosmologists use. It fits the data amazingly well, but it breaks and gives meaningless results right at the start. So Cai and W-E use a slightly modified model where at extremely high energy density gravity is dominated by quantum effects and repels instead of attracts, so as you push back in time you get a bounce.. Or looked at the other way, there is a collapsing phase of the same universe with the same parameters, that rebounds and starts the expansion. CDM stands for "cold dark matter" and Lambda stands for the cosmological constant that appears in Einstein's GR equation (basic to cosmology)

You seem open to the idea. So you might enjoy the Cai&Wilson-Ewing paper. Just read the non-technical parts, if you like---the beginning introduction and the conclusions at the end.

If you google "LambdaCDM bounce" the first hit you get is

http://arxiv-web3.library.cornell.edu/abs/1412.2914v2

they just uploaded an expanded version two days ago, on 28 Jan. I wonder if it is the same as what you get when you click on the standard link for their december 2014 paper:

http://arxiv.org/abs/1412.2914

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Chalnoth

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Mass isn't a conserved quantity, though. It's created and destroyed all the time in high-energy reactions and nuclear decays.In addition to what Bandersnatch said, be advised that this is a complete mis-statement of the law, which is actually called the Law of Conservation of Mass and says that MASS cannot be created or destroyed in a closed system. Matter gets destroyed all the time but the destruction results in no loss of mass since the radiation portion has energy which is equivlent to mass in this context.

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marcus

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[to Priyank]...You seem open to the idea. So you might enjoy the Cai&Wilson-Ewing paper. Just read the non-technical parts, if you like---the beginning introduction and the conclusions at the end.

If you google "LambdaCDM bounce" the first hit you get is

http://arxiv-web3.library.cornell.edu/abs/1412.2914v2

they just uploaded an expanded version two days ago, on 28 Jan. I wonder if it is the same as what you get when you click on the standard link for their december 2014 paper:

http://arxiv.org/abs/1412.2914

Yes! I checked and the usual link already gets the new expanded version of the paper. Here is the brief one paragraph summary, with a link to the full PDF text:

http://arxiv.org/abs/1412.2914

Yi-Fu Cai, Edward Wilson-Ewing

(Submitted on 9 Dec 2014 (v1), last revised 28 Jan 2015 (this version, v2))

We study a contracting universe composed of cold dark matter and radiation, and with a positive cosmological constant. As is well known from standard cosmological perturbation theory, under the assumption of initial quantum vacuum fluctuations the Fourier modes of the comoving curvature perturbation that exit the (sound) Hubble radius in such a contracting universe at a time of matter-domination will be nearly scale-invariant. Furthermore, the modes that exit the (sound) Hubble radius when the effective equation of state is slightly negative due to the cosmological constant will have a slight red tilt, in agreement with observations. We assume that loop quantum cosmology captures the correct high-curvature dynamics of the space-time, and this ensures that the big-bang singularity is resolved and is replaced by a bounce. We calculate the evolution of the perturbations through the bounce and find that they remain nearly scale-invariant. We also show that the amplitude of the scalar perturbations in this cosmology depends on a combination of the sound speed of cold dark matter, the Hubble rate in the contracting branch at the time of equality of the energy densities of cold dark matter and radiation, and the curvature scale that the loop quantum cosmology bounce occurs at. Importantly, as this scenario predicts a positive running of the scalar index, observations can potentially differentiate between it and inflationary models. Finally, for a small sound speed of cold dark matter, this scenario predicts a small tensor-to-scalar ratio.

Comments: 14 pages, 8 figures, v2: Discussion extended and references added

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The total mass-energy in a closed system is conserved. When the mass is destroyed, the radiant energy counts into the sum, right?Mass isn't a conserved quantity, though. It's created and destroyed all the time in high-energy reactions and nuclear decays.

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Chalnoth

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No. Definitely not. A nuclear decay will typically lower the mass-energy of a system, while many high-energy collisions will add to the mass-energy. In these systems, the mass-energy is converted to/from kinetic energy.The total mass-energy in a closed system is conserved. When the mass is destroyed, the radiant energy counts into the sum, right?

It is possible to say that energy is conserved

But when you've got a curved space-time, much of the time it's impossible to say that energy is conserved across the entire system, regardless of whether it's closed or not. As Bandersnatch pointed out, global energy conservation just doesn't work, because there isn't a unique definition of total energy for a curved space-time.

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OK, I had this wrong. Thanks for straightening me out.No. Definitely not. A nuclear decay will typically lower the mass-energy of a system, while many high-energy collisions will add to the mass-energy. In these systems, the mass-energy is converted to/from kinetic energy.

It is possible to say that energy is conservedlocally. What this means is that it's possible to use equations such that the change in energy in an infinitesimally-small region is equal to the energy flowing into that region. The caveat here is that this only works reliably for flat space-time, but since you can always use coordinates where the local space-time is flat, this is always possible to do locally. This is why we generally consider energy to be conserved in reactions on the Earth: the space-time curvature is small, and regardless we typically deal with reactions that are tiny in size. Furthermore, we can write down a gravitational potential energy which allows us to account for the energy changes induced by gravity (this isn't always possible).

But when you've got a curved space-time, much of the time it's impossible to say that energy is conserved across the entire system, regardless of whether it's closed or not. As Bandersnatch pointed out, global energy conservation just doesn't work, because there isn't a unique definition of total energy for a curved space-time.

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mfb

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The total energy stored in particle masses is not conserved, but the total energy in a closed system is, and seen as "closed box" the mass you would assign to this system in its rest frame stays the same even with nuclear decays. This is not an exotic model - we do this all the time when we talk about the mass of protons which is mainly QCD binding energy.

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Yes, this is what I was talking about.

The total energy stored in particle masses is not conserved, but the total energy in a closed system is, and seen as "closed box" the mass you would assign to this system in its rest frame stays the same even with nuclear decays. This is not an exotic model - we do this all the time when we talk about the mass of protons which is mainly QCD binding energy.

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RUTA

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Robert Geroch, General Relativity from A to B, University of Chicago Press, Chicago, 1978.

“There is no dynamics within space-time itself: nothing ever moves therein; nothing happens; nothing changes.”

With this view, the existence of the BB is no more mysterious than any other location on the spacetime manifold, e.g., me writing this post.

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