For any, like me, wondering "how close"? 5 minutes of naive googling and extrapolation about the "z = 0.151" (which I learned refers to redshift) leads me to the answer ~2B light years. For reference, the diameter of the Milky Way is a bit over 100k light years.
> "Spectroscopic observations of the afterglow and the host galaxy provided a redshift of z = 0.151 (de Ugarte Postigo et al. 2022; Castro-Tirado et al. 2022; Izzo et al. 2022), corresponding to a distance of 749.3 Mpc."
3.262e6 (so about 3.3 million light years per megaparsec) * 749.3 gives 2.445 billion light years, so in the ballpark.
The key thing in getting these estimates is the complex business of standard candles, which are astronomical objects believed to have a well-defined luminosity so one can calibrate distances by looking at how faint they are. There are two major kinds used, Cepheid variables (which only cover the range from 1 kpc to 50 Mpc, so not useful here) and type 1a supernovae (which are rare):
> "Beyond the question of whether conditions in the early universe affect the luminosity of a type 1a supernova, a second problem besets this standard candle that severely limits its use: supernovae are rare in any given galaxy. They therefore cannot be used to determine the distance to any galaxy that we may be interested in. Their only uses are in calibrating other distance measures, such as the cosmological redshift, in testing cosmological theories, and in studying the surrounding of these supernovae. Current studies with these standard candles are examining the expansion of the universe at redshifts between z = 0.01 and 1."
My exceptionally naive understanding is that "how long ago" and "how far away" aren't really things that cosmologists talk about.
Because of relativity and the expansion of space neither really has a meaning. It's like the idea of common time: when did something happen on Alpha Centauri? You can't really answer that. Light cones and spheres of influence and so on. "When" is only a valid question when the thing you're asking about isn't relatavistically far away. Deep stuff.
Source: I just read Carlo Rovelli's The order of time. Would be overjoyed if someone who knows what they're talking about would correct/enhance this response. :-)
It's perfectly possible to say when things happened at Alpha Centauri. Relativity doesn't really start messing with the notion of simultaneity until you're in a different reference frame, i.e. moving at a different speed from the observer you're comparing yourself to. But if the only difference is distance, and you know the distance, you really can just run the speed of light backward and get a meaningful time in your own past for a distant event you're just now getting the light from. Think of it this way: since you're moving the same speed, there's no time dilation between your clocks, so you can safely line up ticks between your clock and a hypothetical clock far away on Alpha Centauri or whatever.
I assume things get a little more interesting at big redshifts, because you really are moving at relativistic speeds wrt what you're observing. I don't know off hand how they handle that. I will say I'm pretty sure the reason cosmologists tend to talk in redshifts is that redshifts can be pretty much directly measured and actual distance is... tricky. If you haven't already looked into the "cosmic distance ladder" you're likely to find it interesting.
Source: we covered this stuff in a college class. My math is very rusty, but I did do the math.
> if the only difference is distance, and you know the distance,
But are we not moving in relation to Alpha Centauri at a fabulous speed, because all the galaxies are moving one way or the other and rotating as is Earth moving around the Sun and rotating around its axis?
No, Alpha Centauri and Sol are both in orbit, in the same direction, around the Milky Way. So I mean, the relative speed between us and Alpha Centauri is probably pretty big compared to a car, but it's just noise in our orbital velocities, which are still peanuts compared to c.
> "how long ago" and "how far away" aren't really things that cosmologists talk about.
In actual textbooks and technical papers, like this one, yes, that's correct; they don't talk about them because they're not directly observable, and deriving them from the things that are directly observable is model dependent. So they prefer to talk about the things that are directly observable, of which the key ones are redshift, apparent luminosity, and (for objects that are large enough, like galaxies) angular size.
The relativistic effects you mention are additional reasons why those things are not usually talked about in technical writings.
Pop science writings are another matter: many cosmologists do talk about how long ago and how far away in those contexts without taking the time to add the caveats about how we don't directly observe those things.
I wonder how often things we observe are actually farther away than they seem, they're just rapidly moving towards us relative to their surroundings, or vice versa.
Seems like it's totally possible that something could be 4 billion light years away, or 8 billion light years away, and have the same red shift. Maybe sometimes gravitational lensing would help determine if a star is behind or in front of another? I wonder if it's pretty common to observe objects that are more blue shifted than something that is clearly closer to us.
> Seems like it's totally possible that something could be 4 billion light years away, or 8 billion light years away, and have the same red shift.
In theory, of course this is possible. But in our actual universe, that's not what happens. In our actual universe, there is a close relationship between how far away something is and its redshift. Or, to phrase it in terms of direct observables, in theory we could have an object that was 4 billion light years away and an object that was 8 billion light years away, both with the same redshift--but they would be very different in terms of other observables like apparent luminosity and angular size. So in the general case, in theory, we would not expect any particular relationship between those variables.
But in our actual universe, there is a close relationship between all three of those variables: redshift, apparent luminosity, and angular size all vary in concert and are closely correlated. That kind of observation is what leads cosmologists to conclude that our universe is expanding.
> I wonder if it's pretty common to observe objects that are more blue shifted than something that is clearly closer to us.
It's not only not "pretty common", it never happens except for objects very close to us, cosmologically speaking--for example, the Andromeda galaxy is somewhat blueshifted, indicating that it is moving towards us, while the Magellanic Clouds, which are closer (they are satellite galaxies of our own galaxy), are not, AFAIK, blueshifted. But all of these objects are in the same local, gravitationally bound group as our galaxy, so their relative velocities don't tell us anything useful about the large scale behavior of the universe.
Is it more accurate to interpret inferred distances derived from redshift as an average estimate? If so it would be great if popular science reporting also included the confidence intervals
> Is it more accurate to interpret inferred distances derived from redshift as an average estimate?
We don't estimate distances just based on redshift. We use all three of the observables I mentioned--redshift, apparent luminosity, and angular size--as inputs to estimate parameters in our cosmological models, and one of those parameters is the scale factor--the "size" of the universe, which is what is used in the model to ultimately get actual distances--as a function of time. The models of course have error bars--no model is ever exact--and some of those error bars are due to variations of the motions of individual galaxies from the idealized cosmological expansion in the models. So in that sense distances given by the models are a sort of estimate, yes, based on the average motion.
The effect you are talking about is real, but it's generally small. The cause is called peculiar velocity, and it could introduce uncertainties of a few percent, which is relatively small for cosmological standards. We know how velocities are distributed inside galaxy clusters. It leads to the "finger of god" effect, which makes it appear as if objects would be distributed such that they tend to align along our line of sight.
Yes, in my 5min review it came up very quickly that “distance” and “light years” have limited uses in astrophysics.
In the abstract, they reference the “z” variable of redshift in light (that has traveled for billions of years during which cosmic expansion continued and expanded the original distance) and kiloparsecs (referring to the much closer span of Milky Way through which the emissions traveled, from which they are drawing inferences about dust density therein).
Large scale universe surveys rarely bother with light years as such, they're interested in the structure of the universe itself since birth and are looking to the properties of entire galaxies so far distant individual stars defy resolution (for the most part, stars with energetic burst behaviour stand out).
EG:
Planned Optical Srveys
The Hector Galaxy Survey
The Hector Galaxy Survey will use the Hector-I instrument on the AAT to collect resolved spectroscopy for 15,000 galaxies out to z < 0.1.
The Taipan galaxy survey
It will use the TAIPAN facility on the UK Schmidt Telescope at Siding Spring Observatory to collect spectroscopy for more than a million galaxies in the local Universe (z<0.3) that are brighter than i<17.
At this scale, you have to specify which distance measure you are using. I can infer that you mean light-travel distance, but a cosmologist would use the comoving distance unless specified otherwise. That's what the other comment was getting at with the 90B light-years and the observable universe.
In articles for the general population, you're right in both cases. For stars, you are also right in general. Well, we'd both be right, because for pretty much all observable stars light travel distance and comoving distance are equal (doesn't really make sense to talk about comoving distances or cosmological redshifts within a galaxy). In scientific populations, distances to galaxies are always given either directly in redshifts or perhaps in special cases in comoving distances.
The observable universe is roughly 90B LY in diameter, which factors in how much the universe expanded the source of the light (or rather radio signal) away since its emission.
Neil Gehrels Swift Observatory, previously called the Swift Gamma-Ray Burst Explorer, is a NASA three-telescope space observatory for studying gamma-ray bursts (GRBs) and monitoring the afterglow in X-ray, and UV/Visible light at the location of a burst.
It was launched on 20 November 2004, aboard a Delta II launch vehicle.
Headed by principal investigator Neil Gehrels until his death in February 2017, the mission was developed in a joint partnership between Goddard Space Flight Center (GSFC) and an international consortium from the United States, United Kingdom, and Italy.
The mission is operated by Pennsylvania State University as part of NASA's Medium Explorer program (MIDEX).
In the past 28 days, Swift has received 88 Target of Opportunity requests (3.1 per day), from 63 members of the community, for 82 different celestial objects.
In that time Swift observed 30.3 targets in 113.8 separate observations on average per day.
Swift was observing for 71.2% of the time, the rest of the time spent slewing or passing through the South Atlantic Anomaly.
Since launching in November 2004, Swift has performed 653,651 observations.
> The South Atlantic Anomaly (SAA) is an area where Earth's inner Van Allen radiation belt comes closest to Earth's surface, dipping down to an altitude of 200 kilometres (120 mi). This leads to an increased flux of energetic particles in this region and exposes orbiting satellites (including the ISS) to higher-than-usual levels of ionizing radiation.
Or, more seriously, Magnetorquers for sat attitude control and stabilization are somewhat obscure given they rely on good knowledge of the earth's geomagnetic field which fluctuates daily.
Science is the only news, all the rest is just politics - Stewart Brand
The world of GRB's and the interpretation of such highly energetic phenomena was decidedly speculative just a few decades ago, but increasingly gravitational waves, horizon telescopes etc pin down the fantastic world of general relativity with its spacetime singularities and other oddities as actually... real :-)
Just like quantum mechanics its quite hard to internalize these concepts. I suppose if we can somehow fix "politics" we'll have all the spacetime in the world to work things out.
The International Astronomical Union (IAU) and the American Astronomical Society (AAS) routinely issue notes on units and insist that ergs are "deprecated".
Various astrophyicists are hardwired by early adoption to to use CGS electromagnetic units and don't feel obliged to follow no union rules . . . no one rejects their papers over units, so <shrug>.
A foe is a unit of energy equal to 1044 joules or 1051 ergs, used to express the large amount of energy released by a supernova.[1] An acronym for "[ten to the power of] fifty-one ergs",[2] the term was introduced by Gerald E. Brown of Stony Brook University in his work with Hans Bethe, because "it came up often enough in our work".[3]
Would love to be informed how wrong I got it!