The Last of the Universe’s Ordinary Matter Has Been Found(quantamagazine.org) |
The Last of the Universe’s Ordinary Matter Has Been Found(quantamagazine.org) |
sounds like the work to rule out false-positives would be huge. This is putting a lot of weight on a technique that is not fully described in the paper (i might have missed, just glanced at them for now, and i am an amateur that just like to fiddle with similar data).
The third team took a different approach [1], with results that are both more accurate, less prone to FP, and generally agree with the other findings.
> Here we report observations of two absorbers of highly ionized oxygen (O VII) in the high-signal-to-noise-ratio X-ray spectrum of a quasar at a redshift higher than 0.4. These absorbers show no variability over a two-year timescale and have no associated cold absorption, making the assumption that they originate from the quasar’s intrinsic outflow or the host galaxy’s interstellar medium implausible.
After my understanding of the stacking technique as used in other contexts in astrophysics and without going into too much details :
The problem they were facing is that the signal to noise of the images of the filaments was too low to say that they had detected anything in any individual images. However by stacking (adding) images they were able to detect it because the signal grows roughly with N (N being the number of images) and the noise grows with sqrt(N). So by stacking enough images you'll get the signal to noise necessary to say you've detected sth.
Now, my hypothesis is that "between all two bright lights, there is third, dimmer one, hidding". And then i prove it by filtering X from the two bright light halo, and prove that Y is left proving that the third light is there.
Now, how can i be sure Y is really a third dimmer light? and not just noise on the function i used to try to clean up the halo of the two bright light?
I notice that Oumuamua happened to pass within some 20 million km of earth within a decade or so of having systems in place to spot it. Wouldn't this imply there are an awful lot of them?
Either way, the energy required to maintain operations anywhere near Jupiter means you probably want to find your hydrogen somewhere else.
Seems like we are somewhat validating Vernor Vinge's "zones of thought" idea
I'm not really good at physics at this level so it throws me off. It makes it very difficult to really understand what they are talking about.
You would think physicists would be very precise with their language, by I guess they mostly write for people who are know what they are talking about.
edit: excuse my french ;)
[t]he energy released by a cosmic collision increases as the
square of the incoming object's speed, so a comet could pack
nine times more destructive power than an asteroid of the
same mass. (https://www.space.com/26264-asteroids-comets-earth-impact-risks.html)
With a single observation we can't deduce much of anything concrete except to floor the incidence of these interstellar objects at greater than 0. I'm no astronomer, but I assume models of interstellar objects as they reflect actual risk to Earth wouldn't be very useful without more observations. Whatever the average density in galactic space, I'm betting they're not uniformly distributed. Our solar system is speeding through space that could be littered with clouds of objects.[1] Are we entering a cloud? Leaving a cloud? We can't know without more observations.
[1] There are theories that posit that the ~30- and ~225-million year cycles we see in extinction events are a function of our solar system's orbit in the galaxy, which takes about 200-250 million years. Shorter cycles could relate to the inclination of our orbit (and other stars' orbits) relative to the galactic plane.
The kinetic energy of objects is proportional to the square of the velocity (Ke = (mv^2)/2), so an object going 4 times faster than a solar system object has 16 times the energy for the same mass. This makes it possible to have extinction level events from rocks that are 1/4 the size of planet killing asteroids.
Oumuamua interstellar asteroid. 230x35x35m, ~= 280000 m^3
Density assumption: 2 x water. => mass is ~500,000 metric tonnes.
Spotted only after passing the Sun. Assume we'd spot such objects only if they came within the orbit of mercury so are well illuminated. Assume one such object every 10 years (we've not been searching very long with automated telescopes), and we spot all of them.
Mean mercury orbit radius ~ 60,000,000 km
Area of mercury's orbit: 1.1 x 10^16 km^2
Mercury's orbital area x path length in 10 years = volume swept by one visible object in 10 years.
Asteroid velocity ~100,000 km/h
Path length in 10 years = 100,000 x 10 x 24x365. Swept volume ~ 10 x 10^25 km^3
Distance to Alpha Centauri: 4.37 light years = 4.37 x 9.5 x 10^12 km = 4.15 x 10^13km
Sol's "cube of influence" ~= 7 x 10^40 km^3
Cube of influence / swept volume = rough estimate of number of asteroids in cube of influence. Number of asteroids: 7 x 10^14
Mass of asteroids: 3.5 x 10^20 tonnes. Mass of sun: 2 x 10^27 tonnes.
Conclusion: dark interstellar asteroids like Oumuamua are a tiny fraction of the visible mass of the galaxy.
Finding one in a decade's span within 20 million km would imply there are "an awful lot of them"?
It's an observation, so it sets some level of constraints on the rate. Though it's true that an estimate of that rate would have large uncertainties.
[1] https://arxiv.org/abs/1805.04555
Now we can get to discuss the third paper ;)
Not quite: According to the cosmological standard model, the visible universe will continue to grow (ie new galaxies will continue to come into view) - but only asymptotically, ie until a maximum size given by the comological event horizon is reached. However, the parts of the universe that aren't gravitationally bound to us will become fainter and fainter and increasingly resdhifted, and eventually, we'll be unable to detect other theoretically visible galaxies due to technological limitations.
By definition, infinite the universe is not.
And
Infinite by definition, the universe is not.
There's a cosmological event horizon. Light emitted from within will reach us in finite time, light emitted from without won't. Similar to how a distant observer will never see on object falling into a (stationary) black hole cross the Schwarzschild horizon, we won't see galaxies crossing the cosmological horizon.
To me, it would seem that were it finite, there would be a point at which one would look back, and see the galaxy and clusters that compose the universe; forward would be an expanse of nothingness. But if this isn't the case, then how I can keep progressing forward (presumably forever, as I can't hit an edge) through space, encountering galaxy after galaxy, but it is still finite?
Unless this is like RPG games where the edges wrap.
Good question, but find me the edge of an idealized balloon. Where is the edge of a sphere? As to why you won’t come back, remember that spacetime is expanding faster an faster, and you can only travel below light speed. Mind you that’s just one possibility. The universe at large could be a lot of different things, but as humans were causally disconnected from anything beyond the shrinking observable universe. Shrinking from our perspective at least, because of the aforementioned expansion and speed limit.
The cosmological event horizon is the light cone at future infinity and the asymptotic boundary of the observable universe: Light emitted within the horizon will take a finite time to reach us, whereas light emitted right at the horizon would take an infinte amount of time to arrive; in a way, light emitted beyond the horizon still moves towards us in the sense that the comoving distance decreases, but we'd have to wait a longer-than-infinte amount of time for it to arrive...
Oh, and by the way, an expansion faster than the speed of light is consistent with relativity. Special relativity only describes local laws of physics -- you and the edge are not "local". And general relativity doesn't have a constraint on a maximum velocity between 2 arbitrary points in spacetime.
Limiting discussion to the visible universe (to a "finite" universe by eliding messy details) can mislead by creating seeming contradictions. It's sort of like saying that evolution doesn't exist (as a first order approximation) because the lineage from ape to man is just too complex and doesn't really matter; let's simplify things by eliding that lineage so we have an easier time analogizing human morphology and genetics as it relates to practical questions. It can work superficially but even laymen will have a sense that things don't add up, not to mention that it doesn't help resolve the more important "big" questions often implicit in any discussion.
The fact that the universe is likely infinite stems from experimental results confirming topological characteristics that reflect infinite space. Fortunately, when you try to conceptualize phenomena like the Big Bang, a flat, open infinite universe actually makes things simpler, IMO.
Now what lies beyond the universe's boundary surface? Is such a surface even present?