4 ms·

You are right but this has already been done using the CMB data and the universe seems flat with a 0.4% uncertainty. I don't think there is any need for artific

You are right but this has already been done using the CMB data and the universe seems flat with a 0.4% uncertainty. I don't think there is any need for artificial probes for it. Source: https://arxiv.org/abs/1502.01589 https://arxiv.org/abs/1502.01589

ars 7y ago
That's exactly the point of the article - the CMB data is far less clear than advertised.
- namirez 7y ago
  Fair enough, but better CMB data will resolve the issue at some point. It's not that we need a totally new way of measuring the space curvature.
  - ars 7y ago
    CMB data is based on modeling and simulation. Historically that type of science has never been accurate because there are always assumptions baked into the model - but what if the assumptions are wrong? The CMB results assume dark matter and dark energy are real, it assumes a specific type of big bang theory. But none of those things have direct evidence. We absolutely do need direct measurement of this. And this is an especially bad type of modeling because we only have a single example. (In contrast to say, weather, where we can keep refining the model and comparing to the real world.)
    - namirez 7y ago
      > what if the assumptions are wrong? Dark matter and dark energy are not assumptions. The assumptions are the general relativity and the FLRW metric which is the solution of Friedmann equations. The assumptions of Friedmann equations are that the universe is homogeneous, isotropic, and a perfect fluid. A perfect fluid means the universe has only a local density and pressure. There are no other assumptions in deriving the FLRW metric. Even if were to verify these result by direct measurements, we wouldn't be able to send a probe across the universe. Please note that local curvature in the Milky Way or even the Virgo cluster wouldn't say anything about the curvature of the universe at very large scales. We are talking about curvature at scales larger than 100Mpc (more than 300 million light years).
    - raattgift 7y ago
      The CMB was (and is) observed a lot: https://en.wikipedia.org/wiki/List_of_cosmic_microwave_background_experiments https://en.wikipedia.org/wiki/List_of_cosmic_microwave_backg... The CMB is the evidence for a hot big bang (note, this is not the same as a singularity) although of course observational cosmologists want to trace back from the first starlight to the CMB and have various plans to do so. Surprises in that "dark age" are more than welcome because they are bound to reveal things about low-density momentum-energy. > assume dark matter and dark energy are real Nobody has put together a self-consistent physical cosmology model that reasonably matches actual observations of the shape, scatter, and shift of galaxies across our sky, without the theory containing some additional nonluminous tension and pressure terms that take effect earlier than the surface of last scattering and persist to the present day. The self-consistency problems are expected to blow apart consistency with observation. (Details in e.g. Famey & McGaugh 2011 §9). The need for a tension term to match observations arose even in 1917 [1], and the more detailed 1929 Hubble observations supported the idea of a constant tension independent of visible sources; that is what modern observations still support. The need for an additional pressure term became obvious from the 1960s Rubin observations; the term was pretty simple initially and that simplicity was well-supported through the 1980s and 1990s. One could through that period support theories wherein the pressure term appeared everywhere luminous sources appeared in some straightforward function (e.g. Milgrom 1983). Modern observations have several advantages in the observable details, and those support a rather more complicated interplay between luminous sources and the pressure term, particularly at scales much larger than kiloparsecs. If you can find a theoretical mechanism that produces the observed small anisotropies of the CMB without introducing some tension and pressure acting on an almost entirely homogeneous photon gas with an almost perfect (albeit highly redshifted) blackbody spectrum, well, that'd be amazing. It should of course also be compatible with the actual observations of less-highly-redshifted molecular gas spectra (and the Lyman-alpha forest), and how those lense around rather than through radio-clumpy foreground galaxy clusters. There are a lot of galaxies in the sky, so many that we've discovered that their shapes are pretty regular across an enormous redshift-range. That's additional observational evidence for the spatial flatness of the universe. Even fairly gentle spatial curvature would distort the shapes of face-on spirals at high redshifts, and that is not what we see. Even the Hubble Ultra Deep Field image imposes pretty significant constraints. It's littered with galaxies and with goodies like the Gravitational Lens Galaxy Cluster CL0024+1654, which have been imaged in various wavelengths by both ground and space telescopes. The match between the small anisotropies in the CMB and galaxies at increasing redshift strongly ties the hands of speculative theorists. Those are the people who do the "modelling and simulation". They are stuck having to match actual observation, assuming their goal is to theorize about the single example we inhabit. Bolshoi and MultiDark had to match Sloan, not the other way around. > bad type of modelling Whose word are you taking for this? > we only have a single example. In contrast to ... weather .. where we can keep ... comparing to the real world As said at the top, we've kept comparing the scatter and shape of galaxies and the scatter and shape of fluctuations in the CMB. How is dealing with evidence questioning the standard value of the H_0 parameter not "refining the model"? Also, today we only have a single example of an Earthlike planet's weather data. And until the late 1960s at the earliest we had weather data of any sort for only one planet. So was the numerical weather modelling work done in the 1950s (e.g. by Charney, von Neumann et al. on ENIAC) also "an especially bad type of modelling"? - -- [1] E Schrödinger, “Über ein Lösungssystem der allgemein kovarianten Gravitationsgleichungen”, Physikalische Zeitschrift,19, 20-22 (1918) cf. https://arxiv.org/abs/1211.6338 https://arxiv.org/abs/1211.6338

ars 7y ago

That's exactly the point of the article - the CMB data is far less clear than advertised.

namirez 7y ago
Fair enough, but better CMB data will resolve the issue at some point. It's not that we need a totally new way of measuring the space curvature.
- ars 7y ago
  CMB data is based on modeling and simulation. Historically that type of science has never been accurate because there are always assumptions baked into the model - but what if the assumptions are wrong? The CMB results assume dark matter and dark energy are real, it assumes a specific type of big bang theory. But none of those things have direct evidence. We absolutely do need direct measurement of this. And this is an especially bad type of modeling because we only have a single example. (In contrast to say, weather, where we can keep refining the model and comparing to the real world.)
  - namirez 7y ago
    > what if the assumptions are wrong? Dark matter and dark energy are not assumptions. The assumptions are the general relativity and the FLRW metric which is the solution of Friedmann equations. The assumptions of Friedmann equations are that the universe is homogeneous, isotropic, and a perfect fluid. A perfect fluid means the universe has only a local density and pressure. There are no other assumptions in deriving the FLRW metric. Even if were to verify these result by direct measurements, we wouldn't be able to send a probe across the universe. Please note that local curvature in the Milky Way or even the Virgo cluster wouldn't say anything about the curvature of the universe at very large scales. We are talking about curvature at scales larger than 100Mpc (more than 300 million light years).
  - raattgift 7y ago
    The CMB was (and is) observed a lot: https://en.wikipedia.org/wiki/List_of_cosmic_microwave_background_experiments https://en.wikipedia.org/wiki/List_of_cosmic_microwave_backg... The CMB is the evidence for a hot big bang (note, this is not the same as a singularity) although of course observational cosmologists want to trace back from the first starlight to the CMB and have various plans to do so. Surprises in that "dark age" are more than welcome because they are bound to reveal things about low-density momentum-energy. > assume dark matter and dark energy are real Nobody has put together a self-consistent physical cosmology model that reasonably matches actual observations of the shape, scatter, and shift of galaxies across our sky, without the theory containing some additional nonluminous tension and pressure terms that take effect earlier than the surface of last scattering and persist to the present day. The self-consistency problems are expected to blow apart consistency with observation. (Details in e.g. Famey & McGaugh 2011 §9). The need for a tension term to match observations arose even in 1917 [1], and the more detailed 1929 Hubble observations supported the idea of a constant tension independent of visible sources; that is what modern observations still support. The need for an additional pressure term became obvious from the 1960s Rubin observations; the term was pretty simple initially and that simplicity was well-supported through the 1980s and 1990s. One could through that period support theories wherein the pressure term appeared everywhere luminous sources appeared in some straightforward function (e.g. Milgrom 1983). Modern observations have several advantages in the observable details, and those support a rather more complicated interplay between luminous sources and the pressure term, particularly at scales much larger than kiloparsecs. If you can find a theoretical mechanism that produces the observed small anisotropies of the CMB without introducing some tension and pressure acting on an almost entirely homogeneous photon gas with an almost perfect (albeit highly redshifted) blackbody spectrum, well, that'd be amazing. It should of course also be compatible with the actual observations of less-highly-redshifted molecular gas spectra (and the Lyman-alpha forest), and how those lense around rather than through radio-clumpy foreground galaxy clusters. There are a lot of galaxies in the sky, so many that we've discovered that their shapes are pretty regular across an enormous redshift-range. That's additional observational evidence for the spatial flatness of the universe. Even fairly gentle spatial curvature would distort the shapes of face-on spirals at high redshifts, and that is not what we see. Even the Hubble Ultra Deep Field image imposes pretty significant constraints. It's littered with galaxies and with goodies like the Gravitational Lens Galaxy Cluster CL0024+1654, which have been imaged in various wavelengths by both ground and space telescopes. The match between the small anisotropies in the CMB and galaxies at increasing redshift strongly ties the hands of speculative theorists. Those are the people who do the "modelling and simulation". They are stuck having to match actual observation, assuming their goal is to theorize about the single example we inhabit. Bolshoi and MultiDark had to match Sloan, not the other way around. > bad type of modelling Whose word are you taking for this? > we only have a single example. In contrast to ... weather .. where we can keep ... comparing to the real world As said at the top, we've kept comparing the scatter and shape of galaxies and the scatter and shape of fluctuations in the CMB. How is dealing with evidence questioning the standard value of the H_0 parameter not "refining the model"? Also, today we only have a single example of an Earthlike planet's weather data. And until the late 1960s at the earliest we had weather data of any sort for only one planet. So was the numerical weather modelling work done in the 1950s (e.g. by Charney, von Neumann et al. on ENIAC) also "an especially bad type of modelling"? - -- [1] E Schrödinger, “Über ein Lösungssystem der allgemein kovarianten Gravitationsgleichungen”, Physikalische Zeitschrift,19, 20-22 (1918) cf. https://arxiv.org/abs/1211.6338 https://arxiv.org/abs/1211.6338