5 ms·

Its 12 billion light years away. Just keep in mind that this is what it looked like 12 billion years ago => it takes the light 12 billion years to reach the Ear

Its 12 billion light years away. Just keep in mind that this is what it looked like 12 billion years ago => it takes the light 12 billion years to reach the Earth. So the image as seen is really that old!

nightpool 15y ago
Actually, it's better to keep in mind that this picture is an artist's concept, not an actual photo :)
- damoncali 15y ago
  Even the photos take artistic license. More often than not, the spectrum used is out of the visible range, and therefore has to be translated into a visible color scheme.
goodside 15y ago
That's not how it works. If we observe an object that's currently 12 billion lightyears away (i.e., its co-moving distance is 12 billion light years) the light we observe was emitted much less than 12 billion years ago. While light is in transit, both the distances ahead and behind it will increase due to Hubble expansion, so once you get up to cosmic scales like these the normal distance/velocity formulas don't apply. In general, if you point a flashlight into space and wait one year, the distance between you and the photons of light will be slightly higher than one lightyear. As you wait longer, this difference becomes less slight. When you get up to much larger scales (billions of lightyears) the Hubble expansion dominates the equations. In the most extreme cases, you get counterintuitive things popping up like distances that are so large they can't be traversed in any amount of time, even at light speed. Until you're using equations that make that seem natural, you shouldn't rely intuitive calculations from NASA press releases like this. Convincing science journalists of this fact is one of my minor life goals.
- riobard 15y ago
  So what's the actual time it takes for light to travel 12 billion lightyears?
  - goodside 15y ago
    That depends greatly on when in the history of the Universe the light was emitted. The scale factor varies over time. What you probably meant though was "How long did the light that we're seeing from this particular quasar take to get here?" That we can figure out. When you get a problem like this one, the quickest way to get an approximate answer is by interpolating from the following rule of thumb: If we see light today that we know has been in transit since the start of the Universe (15.3 billion years ago), the point it was emitted from is now 46 billion light years away from Earth. If you just assume that the impact of the Hubble expansion is more-or-less proportional to the current distance of the object in question, we can just linearly interpolate between the observed Universe-scale answer and the intuitive Newtonian answer that applies on normal scales. Newtonian answer: 12 billion years Universe-scale answer: 12 Glyr * (15.3 Gyr) / (46 Glyr) = 4 billion years Final approximation: 4 Gyr * (12/46) + 12 Gyr * (1 - 12/46) = 9.9 billion years
    - thyrsus 15y ago
      By "light today that we know has been in transit since the start of the Universe" may I presume you mean the microwave background often referred to as "remnants of the big bang"? Where should I go to understand the derivation of the figure "now 46 billion light years away"? It's not clear to me what you mean by "now" and "away", since time and distance depend on the frame of reference, and it's not clear to me what it means to be "at rest" in an expanding space. "At rest" enters in because we're not going backwards in time, and thus we cannot reach the source of the radiation (the event which generated the radiation at a specific space-time coordinate), but only the event's spatial coordinates, and, absent other specification, those spatial coordinates are "at rest" with respect to our own frame of reference. But again, what does "at rest" mean in expanding space?
      - iwwr 15y ago
- itcmcgrath 15y ago
  Hmm. Given that its distance was more than likely measured by the light, I would assume that 12 billion light-years was the distance when the light was emitted; not its current distance. So, correct me if I'm wrong, that renders your whole point about Hubble expansion moot, no?
  - goodside 15y ago
    Nope. If the quasar were 12 billion light-years away when the light was emitted, the light would never have even reached us! In this scenario, the distance between the quasar and Earth that the light needs to traverse would undergo metric expansion that would hugely outpace any progress made by the light's motion (more precisely, its peculiar motion) in the direction of Earth.

nightpool 15y ago

Actually, it's better to keep in mind that this picture is an artist's concept, not an actual photo :)

damoncali 15y ago
Even the photos take artistic license. More often than not, the spectrum used is out of the visible range, and therefore has to be translated into a visible color scheme.

goodside 15y ago

That's not how it works. If we observe an object that's currently 12 billion lightyears away (i.e., its co-moving distance is 12 billion light years) the light we observe was emitted much less than 12 billion years ago. While light is in transit, both the distances ahead and behind it will increase due to Hubble expansion, so once you get up to cosmic scales like these the normal distance/velocity formulas don't apply. In general, if you point a flashlight into space and wait one year, the distance between you and the photons of light will be slightly higher than one lightyear. As you wait longer, this difference becomes less slight. When you get up to much larger scales (billions of lightyears) the Hubble expansion dominates the equations. In the most extreme cases, you get counterintuitive things popping up like distances that are so large they can't be traversed in any amount of time, even at light speed. Until you're using equations that make that seem natural, you shouldn't rely intuitive calculations from NASA press releases like this. Convincing science journalists of this fact is one of my minor life goals.

riobard 15y ago
So what's the actual time it takes for light to travel 12 billion lightyears?
- goodside 15y ago
  That depends greatly on when in the history of the Universe the light was emitted. The scale factor varies over time. What you probably meant though was "How long did the light that we're seeing from this particular quasar take to get here?" That we can figure out. When you get a problem like this one, the quickest way to get an approximate answer is by interpolating from the following rule of thumb: If we see light today that we know has been in transit since the start of the Universe (15.3 billion years ago), the point it was emitted from is now 46 billion light years away from Earth. If you just assume that the impact of the Hubble expansion is more-or-less proportional to the current distance of the object in question, we can just linearly interpolate between the observed Universe-scale answer and the intuitive Newtonian answer that applies on normal scales. Newtonian answer: 12 billion years Universe-scale answer: 12 Glyr * (15.3 Gyr) / (46 Glyr) = 4 billion years Final approximation: 4 Gyr * (12/46) + 12 Gyr * (1 - 12/46) = 9.9 billion years
  - thyrsus 15y ago
    By "light today that we know has been in transit since the start of the Universe" may I presume you mean the microwave background often referred to as "remnants of the big bang"? Where should I go to understand the derivation of the figure "now 46 billion light years away"? It's not clear to me what you mean by "now" and "away", since time and distance depend on the frame of reference, and it's not clear to me what it means to be "at rest" in an expanding space. "At rest" enters in because we're not going backwards in time, and thus we cannot reach the source of the radiation (the event which generated the radiation at a specific space-time coordinate), but only the event's spatial coordinates, and, absent other specification, those spatial coordinates are "at rest" with respect to our own frame of reference. But again, what does "at rest" mean in expanding space?
    - iwwr 15y ago
itcmcgrath 15y ago
Hmm. Given that its distance was more than likely measured by the light, I would assume that 12 billion light-years was the distance when the light was emitted; not its current distance. So, correct me if I'm wrong, that renders your whole point about Hubble expansion moot, no?
- goodside 15y ago
  Nope. If the quasar were 12 billion light-years away when the light was emitted, the light would never have even reached us! In this scenario, the distance between the quasar and Earth that the light needs to traverse would undergo metric expansion that would hugely outpace any progress made by the light's motion (more precisely, its peculiar motion) in the direction of Earth.