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I always hate it when an article drops that we've "proven the existence of dark matter". We've done no such thing. Usually that sentence describes some of the p

I always hate it when an article drops that we've "proven the existence of dark matter". We've done no such thing. Usually that sentence describes some of the proof of the fact that our matter and energy predictions are off by an order of magnitude... Proof of the problem that dark matter/energy theories attempt to resolve. I know it's a nitpick, but it drives me nuts.

pdonis 9y ago
> we've "proven the existence of dark matter" The article says we've proven the existence of dark energy, not dark matter. They're not the same thing. That said, if it does turn out that the SN1a "standard candle" scale has to be revised, that might impact the finding of accelerated expansion on which the dark energy model is based. It will be interesting to see how other cosmologists respond to this paper.
- sgt101 9y ago
  Do cosmologists allow for the doping of the matter involved in these interaction due to stella aging? It seems to me that more distant supernovas must act differently from the closer ones because they are much younger and therefore undoped by earlier supernova debris.
  - pdonis 9y ago
    As I understand it, the assumption behind using SN1a as standard candles is not that they all have exactly the same absolute brightness, but that there is a reliable relationship between the light curve (the curve of apparent brightness vs. time) and the absolute brightness. This relationship takes into account variation in the chemical composition of the supernova, which is what "doping" would affect. However, what this particular paper seems to be saying is that there might not be just one light curve-brightness relationship, but two, corresponding to two different explosion processes. If that's true, astronomers would need to go back and review the SN1a data with this new classification scheme in mind.
- raattgift 9y ago
  > the finding of accelerated expansion on which the dark energy model is based Dark energy arises because of the decomposition of 4-d spacetime into volumes of 3-d space each distinguishable by a time coordinate, and choosing as the timelike axis the one in which matter is -- on average and at the largest scales -- at rest against the 3-d spatial coordinates. That fixes a preferred frame -- the cosmological frame -- for observers who are comoving with the spatial coordinates. Such observers will see no dipole anisotropy in the cosmic microwave background (or the cosmic neutrino background, or other relic fields), and will be comfortable with approximating the distribution of visible matter far from them as a homogeneous isotropic fluid, in spite of local inhomogeneities and local anisotropies (like, for example, the planet each of us has between her- or himself and a large percentage of the rest of the universe). All comoving observers in such a universe will generally agree that in one timelike direction the visible matter is denser and the CMB is hotter and that in the other timelike direction the visible matter is sparser and the CMB is colder. Let's pick the former direction as the past. All observers in such a universe -- even non-comoving ones -- will agree that it takes energy to separate two massive bodies that are gravitationally bound. So in the past, when comoving matter was denser, it would take more energy to separate the bits of matter from one another than in the present, when comoving matter is sparser. Something is keeping objects like galaxy clusters fixed against the comoving coordinates, instead of giving them peculiar velocities against these coordinates as they gravitationally attract one another.[1] That is dark energy, and we can experimentally determine its value in the cosmological frame. The rate at which visible matter is diluting away along the cosmological frame's past->future direction is what what we try to measure. Standard candles are useful for helping fix a distant galaxy to comoving coordinates. As our confidence in the location of distant matter improves, so does our confidence in the average value of dark energy. There are a sizable number of relationships involving measurable quantities like angular diameter, surface brightness, emission/absorption line "forests", and so forth that help us fix galaxies in the cosmological frame of reference, so throwing questions upon only one coordinate:observable relationship is far from fatal to our estimates of dark energy density. It sure is interesting though! So, back to "accelerated expansion": dark energy represents something working against the tendency of matter to gravitatonally collapse. Whether that something is constant or evolves is something we can determine by observation. Whether that something is uniform everywhere outside galaxy clusters or not is also something we can determine by observation. Current evidence is consistent with a constant uniform feature of the action of large scale structures. In particular, while normally gravitating matter is diluting away in the past->future direction we picked out in our cosmological frame of reference, dark energy is not diluting away. If, still staying in the cosmological frame, we switch our spatial coordinates from comoving coordinates on the cosmological frame to spherical Earth-centric coordinates, we have to adjust our understanding a little. A distant galaxy has a lower radial distance at earlier times than at later times. We can identify this with a recession velocity, and use that to explain the cosmological redshift and the presence of various cosmological horizons. But this is a coordinate-dependent explanation. It is perfectly fine to stick with comoving coordinates and resort to other explanations for the cosmological redshift. Indeed, it was that galaxies at extreme distances still look like galaxies that led to the metric expansion of space in the first place: small-angular-diameter galaxy clusters can't really be moving relativistically away from us or they would look VERY different structurally. Additionally, we haven't abandoned Lorentz covariance, so treating matter receding over the horizon as moving superluminally has to be considered a non-physical artifact of a choice of coordinates and frame of reference. And sure enough, just by changing the coordinates alone, these problems disappear. [1] individual galaxies within a cluster have peculiar velocities against the comoving coordinates that the cluster's centre of momentum stays fixed against. Likewise, massive elements within galaxies (e.g. Earth) have peculiar velocities against the comoving coordinates thanks to local gravitational interactions (e.g. Earth orbits the sun). Each of these introduces an anisotropy in the view of the overall distribution of matter in the universe and in the view of the CMB by an observer with a peculiar velocity. Such observers are not strictly comoving ones. But they're generally awfully close to comoving, because they're almost certainly embedded within a larger structure that is comoving.
greglindahl 9y ago
"proven the existence of dark matter," to me, says that we've proven that there's a thing with some known properties that needs a lot more explaining. And that's the actual intent of the people who are saying it, too. Same thing with dark energy, although even fewer of the properties of dark energy are known.
oldandtired 9y ago
It is interesting that those who are looking experimentally for both dark matter and dark energy have come up empty in every experiment so far performed. The theorists seem to ignore this and rely more on faith in their mathematical theories and models leading to dogmatic reliance on their belief not on experimental evidence. Reminds me of the "The Neverending Debate" between String Theorists and Non-String Theorists. Mathematics is a very useful tool, but if reality differs from the mathematic model then the mathematics is not sufficient and is only a simplistic view for us to use.
- oshepherd 9y ago
  > It is interesting that those who are looking experimentally for both dark matter and dark energy have come up empty in every experiment so far performed. Given that <1% of the dark matter search space has been covered, this is an entirely unsurprising result. The ΛCDM model doesn't come out of theory, it comes out of observation - the only thing we can make fit our observations of the universe is cold dark matter (and a cosmological constant, aka dark energy) The thing a lot of people don't seem to get is that astronomers hate¹ dark energy and dark matter - they're really quite inelegant - but nothing else fits. Every other attempt (and lots and lots of time and energy have been spent on this) to explain our observations without hypothesising the existence of a lot of weakly interacting matter out there (e.g. MOND) has failed to match them ¹ In much the same way physicists dislike the standard model in all of its' ugliness - even though it works
  - oshepherd 9y ago
    (And to put things another way: of course we've come back empty in all searches so far. If we hadn't, we'd stop looking!)
  - oldandtired 9y ago
    The theory doesn't match the observations, so how do we tweak the theory to match the observations - oh wait - let's add dark matter to the theory. Partial fix - oh wait - we need something to add something else to he theory now - let's add ..... dark energy. Oh yeah, this seems to fix the theory to now match observation. The observations do not require dark energy or dark matter to fix things. It is the interpretation of those observations via the theory that requires these entities. Observations are theory neutral. It should be fun to try an come up with a good model/theory and if new observations break the theory, then it should be more fun to see what is a better explanation. For many, many years, I have watched very intelligent people get emotionally caught up in defending their theories and models, when it should be a case of just moving on. They just don't make it fun.

pdonis 9y ago

> we've "proven the existence of dark matter" The article says we've proven the existence of dark energy, not dark matter. They're not the same thing. That said, if it does turn out that the SN1a "standard candle" scale has to be revised, that might impact the finding of accelerated expansion on which the dark energy model is based. It will be interesting to see how other cosmologists respond to this paper.

sgt101 9y ago
Do cosmologists allow for the doping of the matter involved in these interaction due to stella aging? It seems to me that more distant supernovas must act differently from the closer ones because they are much younger and therefore undoped by earlier supernova debris.
- pdonis 9y ago
  As I understand it, the assumption behind using SN1a as standard candles is not that they all have exactly the same absolute brightness, but that there is a reliable relationship between the light curve (the curve of apparent brightness vs. time) and the absolute brightness. This relationship takes into account variation in the chemical composition of the supernova, which is what "doping" would affect. However, what this particular paper seems to be saying is that there might not be just one light curve-brightness relationship, but two, corresponding to two different explosion processes. If that's true, astronomers would need to go back and review the SN1a data with this new classification scheme in mind.
raattgift 9y ago
> the finding of accelerated expansion on which the dark energy model is based Dark energy arises because of the decomposition of 4-d spacetime into volumes of 3-d space each distinguishable by a time coordinate, and choosing as the timelike axis the one in which matter is -- on average and at the largest scales -- at rest against the 3-d spatial coordinates. That fixes a preferred frame -- the cosmological frame -- for observers who are comoving with the spatial coordinates. Such observers will see no dipole anisotropy in the cosmic microwave background (or the cosmic neutrino background, or other relic fields), and will be comfortable with approximating the distribution of visible matter far from them as a homogeneous isotropic fluid, in spite of local inhomogeneities and local anisotropies (like, for example, the planet each of us has between her- or himself and a large percentage of the rest of the universe). All comoving observers in such a universe will generally agree that in one timelike direction the visible matter is denser and the CMB is hotter and that in the other timelike direction the visible matter is sparser and the CMB is colder. Let's pick the former direction as the past. All observers in such a universe -- even non-comoving ones -- will agree that it takes energy to separate two massive bodies that are gravitationally bound. So in the past, when comoving matter was denser, it would take more energy to separate the bits of matter from one another than in the present, when comoving matter is sparser. Something is keeping objects like galaxy clusters fixed against the comoving coordinates, instead of giving them peculiar velocities against these coordinates as they gravitationally attract one another.[1] That is dark energy, and we can experimentally determine its value in the cosmological frame. The rate at which visible matter is diluting away along the cosmological frame's past->future direction is what what we try to measure. Standard candles are useful for helping fix a distant galaxy to comoving coordinates. As our confidence in the location of distant matter improves, so does our confidence in the average value of dark energy. There are a sizable number of relationships involving measurable quantities like angular diameter, surface brightness, emission/absorption line "forests", and so forth that help us fix galaxies in the cosmological frame of reference, so throwing questions upon only one coordinate:observable relationship is far from fatal to our estimates of dark energy density. It sure is interesting though! So, back to "accelerated expansion": dark energy represents something working against the tendency of matter to gravitatonally collapse. Whether that something is constant or evolves is something we can determine by observation. Whether that something is uniform everywhere outside galaxy clusters or not is also something we can determine by observation. Current evidence is consistent with a constant uniform feature of the action of large scale structures. In particular, while normally gravitating matter is diluting away in the past->future direction we picked out in our cosmological frame of reference, dark energy is not diluting away. If, still staying in the cosmological frame, we switch our spatial coordinates from comoving coordinates on the cosmological frame to spherical Earth-centric coordinates, we have to adjust our understanding a little. A distant galaxy has a lower radial distance at earlier times than at later times. We can identify this with a recession velocity, and use that to explain the cosmological redshift and the presence of various cosmological horizons. But this is a coordinate-dependent explanation. It is perfectly fine to stick with comoving coordinates and resort to other explanations for the cosmological redshift. Indeed, it was that galaxies at extreme distances still look like galaxies that led to the metric expansion of space in the first place: small-angular-diameter galaxy clusters can't really be moving relativistically away from us or they would look VERY different structurally. Additionally, we haven't abandoned Lorentz covariance, so treating matter receding over the horizon as moving superluminally has to be considered a non-physical artifact of a choice of coordinates and frame of reference. And sure enough, just by changing the coordinates alone, these problems disappear. [1] individual galaxies within a cluster have peculiar velocities against the comoving coordinates that the cluster's centre of momentum stays fixed against. Likewise, massive elements within galaxies (e.g. Earth) have peculiar velocities against the comoving coordinates thanks to local gravitational interactions (e.g. Earth orbits the sun). Each of these introduces an anisotropy in the view of the overall distribution of matter in the universe and in the view of the CMB by an observer with a peculiar velocity. Such observers are not strictly comoving ones. But they're generally awfully close to comoving, because they're almost certainly embedded within a larger structure that is comoving.

greglindahl 9y ago

"proven the existence of dark matter," to me, says that we've proven that there's a thing with some known properties that needs a lot more explaining. And that's the actual intent of the people who are saying it, too. Same thing with dark energy, although even fewer of the properties of dark energy are known.

oldandtired 9y ago

It is interesting that those who are looking experimentally for both dark matter and dark energy have come up empty in every experiment so far performed. The theorists seem to ignore this and rely more on faith in their mathematical theories and models leading to dogmatic reliance on their belief not on experimental evidence. Reminds me of the "The Neverending Debate" between String Theorists and Non-String Theorists. Mathematics is a very useful tool, but if reality differs from the mathematic model then the mathematics is not sufficient and is only a simplistic view for us to use.

oshepherd 9y ago
> It is interesting that those who are looking experimentally for both dark matter and dark energy have come up empty in every experiment so far performed. Given that <1% of the dark matter search space has been covered, this is an entirely unsurprising result. The ΛCDM model doesn't come out of theory, it comes out of observation - the only thing we can make fit our observations of the universe is cold dark matter (and a cosmological constant, aka dark energy) The thing a lot of people don't seem to get is that astronomers hate¹ dark energy and dark matter - they're really quite inelegant - but nothing else fits. Every other attempt (and lots and lots of time and energy have been spent on this) to explain our observations without hypothesising the existence of a lot of weakly interacting matter out there (e.g. MOND) has failed to match them ¹ In much the same way physicists dislike the standard model in all of its' ugliness - even though it works
- oshepherd 9y ago
  (And to put things another way: of course we've come back empty in all searches so far. If we hadn't, we'd stop looking!)
- oldandtired 9y ago
  The theory doesn't match the observations, so how do we tweak the theory to match the observations - oh wait - let's add dark matter to the theory. Partial fix - oh wait - we need something to add something else to he theory now - let's add ..... dark energy. Oh yeah, this seems to fix the theory to now match observation. The observations do not require dark energy or dark matter to fix things. It is the interpretation of those observations via the theory that requires these entities. Observations are theory neutral. It should be fun to try an come up with a good model/theory and if new observations break the theory, then it should be more fun to see what is a better explanation. For many, many years, I have watched very intelligent people get emotionally caught up in defending their theories and models, when it should be a case of just moving on. They just don't make it fun.