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For some context, Avi Loeb has a certain ... reputation in the astrophysics community. He's a super smart guy, but he has a habit of writing a ton of short, ve

For some context, Avi Loeb has a certain ... reputation in the astrophysics community. He's a super smart guy, but he has a habit of writing a ton of short, very speculative papers. One gets the impression that he's throwing a lot of ideas out there and seeing what sticks. It can be as valid a strategy in astronomy as in the startup world, but the downside is that it can be hard to believe anything he writes. As far as this paper goes, the most interesting implication isn't mentioned in the phys.org article. It seems to me that main idea of the paper is to connect the development of life to the cosmological evolution of the universe. We find that we live in a somewhat special time, when the universe is just starting to be dominated by dark energy after having been dominated by matter since ~300,000 years after the big bang. Anytime you see that we live at some sort of a special time, the Copernican principle leads one to wonder if there is some underlying reason for this. The main argument of this paper is that as the universe becomes more dominated by dark energy, the expansion of the universe accelerates and this shuts off the flow of gas onto galaxies. As galaxies stop accreting matter, star formation slows, making it less likely that life will develop in the far future. However, low mass stars are more abundant and have a longer lifetime, so even if it takes a longer time for life to evolve in that environment, it's still, on balance, more likely that life evolves around low mass stars in the near future ("near" still being tens to hundreds of billions of years). So we would expect that the typical life form will evolve around a low mass star, far in the future. Thus, life on Earth seems to have developed early, but maybe not unusually so, and it's maybe not a coincidence that we happen to live a little after the time at which the universe became dark energy dominated. I would file this away in the "interesting if true" category.

antisthenes 10y ago
> so even if it takes a longer time for life to evolve in that environment, it's still, on balance, more likely that life evolves around low mass stars in the near future ("near" still being tens to hundreds of billions of years). So we would expect that the typical life form will evolve around a low mass star, far in the future. I find that premise to be false, given our (limited) knowledge about exoplanets already. The limited Goldilocks zone for low mass stars usually results in planets being tidally locked, leaving life to develop in narrow bands or in the hostile zones with direct sunlight or lack thereof. Not all exoplanets around faint stars may be tidally locked, but with increasing distance it also pushes them out to the limits, where there may not be enough energy for life to develop (at near freezing ocean temperatures) With more information we may find that perhaps there's also an optimal star brightness, which provides just enough energy at just the right distance for life to develop, disqualifying most dwarf stars. In either case, whatever you believe, given our extremely limited information, any papers produced can be labeled as speculative, especially when we're talking about something where N = 1.
- antognini 10y ago
  Maybe. Many hot Jupiters are probably tidally locked, but it's not clear to me whether rocky planets will be tidally locked. Though given that they're considering timescales of 100s of Gyr maybe they will end up tidally locked by that point. At any rate, I don't think our understanding of tides is good enough to claim that they will or won't be tidally locked one way or another. But needless to say, the authors didn't consider details like this in their paper.
- Retric 10y ago
  Moons are one way around this.
  - WildUtah 10y ago
    I use a moon on my home planet to keep tides interesting and find it lends a pleasing glow to the night sky. Thumbs up.
EGreg 10y ago
We find that we live in a somewhat special time, when the universe is just starting to be dominated by dark energy after having been dominated by matter since ~300,000 years after the big bang. Anytime you see that we live at some sort of a special time, the Copernican principle leads one to wonder if there is some underlying reason for this. I will propose such a puzzle. The moon has been moving away from the Earth at a certain rate every year. Yet it is only in the time frames of the humans that we find the moon and the sun roughly the same size, so they can form full solar eclipses etc. Is this a coincidence? Surely there can't be any physical explanation for this.
- krapp 10y ago
  >Is this a coincidence? Surely there can't be any physical explanation for this. Coincidence seems like a perfectly adequate explanation to me.
  - EGreg 10y ago
    But we are talking about the Copernican principle. Why isn't coincidence a good explanation for ANY such thing?
    - krapp 10y ago
      It is, but I guess that's not interesting.
  - monk_e_boy 10y ago
    The very human response. But it's happening to me, and I'm special, therefore the event must be special.
- srtjstjsj 10y ago
  It's a pretty broad time window: It will take 1billion years for the moon to move 10% farther away. (200K miles 0.1 / (4cm/yr)) https://www.wolframalpha.com/input/?i=(distance+from+earth+to+moon)+*+0.1+%2F+4cm https://www.wolframalpha.com/input/?i=(distance+from+earth+t... Not sure how long back the 4cm/yr rate has been a good estimate.
EGreg 10y ago
This is all speculation. Even the Big Bang theory's extrapolation to say how old the universe is. We simply don't know. Check it out, accounts for Big Bang and dark energy: http://m.phys.org/news/2015-02-big-quantum-equation-universe.html http://m.phys.org/news/2015-02-big-quantum-equation-universe... Some processes in the universe seem to take longer than 13B years http://www.dailygalaxy.com/my_weblog/2010/06/you-couldnt-make-this-up-dept-universe-is-150-billion-years-old-not-137-billion.html http://www.dailygalaxy.com/my_weblog/2010/06/you-couldnt-mak... This seems like the time that Lord Kelvin calculated the Earth to be no more than 100 million years and it took a century for scientists to challenge that. Becauss they discovered radioactivity but mostly because the old generation receded in power.

antisthenes 10y ago

> so even if it takes a longer time for life to evolve in that environment, it's still, on balance, more likely that life evolves around low mass stars in the near future ("near" still being tens to hundreds of billions of years). So we would expect that the typical life form will evolve around a low mass star, far in the future. I find that premise to be false, given our (limited) knowledge about exoplanets already. The limited Goldilocks zone for low mass stars usually results in planets being tidally locked, leaving life to develop in narrow bands or in the hostile zones with direct sunlight or lack thereof. Not all exoplanets around faint stars may be tidally locked, but with increasing distance it also pushes them out to the limits, where there may not be enough energy for life to develop (at near freezing ocean temperatures) With more information we may find that perhaps there's also an optimal star brightness, which provides just enough energy at just the right distance for life to develop, disqualifying most dwarf stars. In either case, whatever you believe, given our extremely limited information, any papers produced can be labeled as speculative, especially when we're talking about something where N = 1.

antognini 10y ago
Maybe. Many hot Jupiters are probably tidally locked, but it's not clear to me whether rocky planets will be tidally locked. Though given that they're considering timescales of 100s of Gyr maybe they will end up tidally locked by that point. At any rate, I don't think our understanding of tides is good enough to claim that they will or won't be tidally locked one way or another. But needless to say, the authors didn't consider details like this in their paper.
Retric 10y ago
Moons are one way around this.
- WildUtah 10y ago
  I use a moon on my home planet to keep tides interesting and find it lends a pleasing glow to the night sky. Thumbs up.

EGreg 10y ago

We find that we live in a somewhat special time, when the universe is just starting to be dominated by dark energy after having been dominated by matter since ~300,000 years after the big bang. Anytime you see that we live at some sort of a special time, the Copernican principle leads one to wonder if there is some underlying reason for this. I will propose such a puzzle. The moon has been moving away from the Earth at a certain rate every year. Yet it is only in the time frames of the humans that we find the moon and the sun roughly the same size, so they can form full solar eclipses etc. Is this a coincidence? Surely there can't be any physical explanation for this.

krapp 10y ago
>Is this a coincidence? Surely there can't be any physical explanation for this. Coincidence seems like a perfectly adequate explanation to me.
- EGreg 10y ago
  But we are talking about the Copernican principle. Why isn't coincidence a good explanation for ANY such thing?
  - krapp 10y ago
    It is, but I guess that's not interesting.
- monk_e_boy 10y ago
  The very human response. But it's happening to me, and I'm special, therefore the event must be special.
srtjstjsj 10y ago
It's a pretty broad time window: It will take 1billion years for the moon to move 10% farther away. (200K miles 0.1 / (4cm/yr)) https://www.wolframalpha.com/input/?i=(distance+from+earth+to+moon)+*+0.1+%2F+4cm https://www.wolframalpha.com/input/?i=(distance+from+earth+t... Not sure how long back the 4cm/yr rate has been a good estimate.

EGreg 10y ago

This is all speculation. Even the Big Bang theory's extrapolation to say how old the universe is. We simply don't know. Check it out, accounts for Big Bang and dark energy: http://m.phys.org/news/2015-02-big-quantum-equation-universe.html http://m.phys.org/news/2015-02-big-quantum-equation-universe... Some processes in the universe seem to take longer than 13B years http://www.dailygalaxy.com/my_weblog/2010/06/you-couldnt-make-this-up-dept-universe-is-150-billion-years-old-not-137-billion.html http://www.dailygalaxy.com/my_weblog/2010/06/you-couldnt-mak... This seems like the time that Lord Kelvin calculated the Earth to be no more than 100 million years and it took a century for scientists to challenge that. Becauss they discovered radioactivity but mostly because the old generation receded in power.