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I have some questions: 1. When they say "70% of the universe is made up of dark energy, the rest is matter and dark matter", in what sense is the universe "ma

I have some questions: 1. When they say "70% of the universe is made up of dark energy, the rest is matter and dark matter", in what sense is the universe "made up" of those things? Or, perhaps, what is the "universe" that is made up of those things? If dark energy is this cosmological constant, in what sense does it "make up" 70% of the universe? 2. If the value of w is changing, and w is this cosmological constant that is dark energy (I'm probably not using the right words here), and dark energy is part of what makes up the sum total of the universe, did the decrease in dark energy go somewhere? Did it show up as an increase of something else? If so, what? 3. If w is changing over time, even if very slowly, should we suspect that other things might also be changing? The speed of light? The fine structure constant? For that matter, if w changes, does that require something else to change as a cause? 4. How did w change? Was it a step function, a corner (discontinuous first derivative), or a smooth change? Is it now constant, or is it continuing to decrease? (Yeah, I know - we just discovered this at all, I'm almost certainly asking for more precision than we have data for yet...)

mr_mitm 2y ago
1. It's the energy content. The stuff you can see (baryonic matter) is 5%, dark matter is 25%, the rest is dark energy. Technically you should take radiation into account, but this is negligible in today's universe. The types of energy can be characterized by how it changes as the universe expanda. If it becomes larger by a factor of a, matter density goes down as a^3, radiation density goes down as a^4 and dark energy density stays constant. Energy is not conserved in an expanding universe, so don't worry about that. 2. W is not the cosmological constant, but a related parameter. It does not go anywhere, the density just changes. 3. These things are unrelated. We do look for changes in these constants either way as good as we can, because imagine the excitement! But so far they all look very constant. 4. No one knows. It's probably always been close to -1, or else the universe would look very different, but you can play around with all kinds of time parameterization of w. The math gets complex very quickly, so the most common parameterization is a linearization. Simply measuring that is a challenge, so measuring this quantity in a model independent way is extremely difficult.

mr_mitm 2y ago

1. It's the energy content. The stuff you can see (baryonic matter) is 5%, dark matter is 25%, the rest is dark energy. Technically you should take radiation into account, but this is negligible in today's universe. The types of energy can be characterized by how it changes as the universe expanda. If it becomes larger by a factor of a, matter density goes down as a^3, radiation density goes down as a^4 and dark energy density stays constant. Energy is not conserved in an expanding universe, so don't worry about that. 2. W is not the cosmological constant, but a related parameter. It does not go anywhere, the density just changes. 3. These things are unrelated. We do look for changes in these constants either way as good as we can, because imagine the excitement! But so far they all look very constant. 4. No one knows. It's probably always been close to -1, or else the universe would look very different, but you can play around with all kinds of time parameterization of w. The math gets complex very quickly, so the most common parameterization is a linearization. Simply measuring that is a challenge, so measuring this quantity in a model independent way is extremely difficult.