Getting the hubble constant from time delay in a gravitational lens

In summary, the conversation revolves around finding the Hubble constant from a time delay in a gravitational lens using data from QSO 0957+561. The key is to determine the time delay by observing the pattern of flux densities and Julian days. Once the time delay is found, the distance can be calculated using the equation for Dls, which depends on the redshifts of the lens and source. The conversation also suggests seeking help in the homework help forum for further guidance on finding the distance and determining the angle between the two images.
  • #1
cegec
5
0
Hi everyone,

First time posting, sorry if this is in the wrong format or wrong place. For a project in a cosmology course I need to find the Hubble constant from from time delay in a gravitational lens. the one we are using is QSO 0957+561. My question is how to I find the time delay from a set of data containing the flux densities (for each of the two images the lens created) and the julian day for each data entry?

thanks
ps please let me know if I need to be more specific in this question.
 
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  • #2
I've seen the QSO 0957+561 lightcurves and especially in the g-band there is this obvious spike and drop-off that occurs earlier in the A source (the direct straightline source) and then a certain number of Julian days later it occurs in the B source, where the light was bent and traveled two sides of a triangle to get to us.

It's obvious by eyeball pattern recognition. You can't miss the pattern. So all you have to do is count the Julian days. And that is the time delay.

Maybe you don't have a visual plot of the lightcurves? So you can't eyeball and see the pattern, and its shifted twin pattern? then my advice would be to make a plot so your visual brain can spot the pattern.

To me the interesting part is after you have determined the delay, which is easy to do, how do you determine from that how far away the quasar is!

After you have found the delay why don't you come back and tell us about it and tell us how you are going to find the distance. Nice project.

Officially your post belongs in homework help forum, not cosmo. They know what to say and not to say there. We shouldn't give away too much in hints. Here is the link to homework help forum:
https://www.physicsforums.com/forumdisplay.php?f=152
If you find you need more help than this you should probably go there.
 
  • #3
thanks marcus that makes perfect sense! i was making it out to be more complicated than it was. how would you use the time delay to find the distance? the equation i have for Dls depends only on the redshift for the lens and the redshift for the source. should i be working with something different? appreciate the help
 
  • #4
I'm curious: do you have the angle between the two images?
I am picturing a skinny triangle. I don't know if that is the right picture.

Ideally you should go to Homework forum and ask the same question. They are skilled at helpful questions without giving away. So then you learn more.

Your real problem is finding DISTANCE from the grav. lensing time delay. If you knew the distance along the straightline path (image A) then you could easily compare that with the redshift and find H.

Your job is to find the distance, along the long straight side of the triangle.
 
  • #5
yes they are 6 arcseconds apart
 
  • #6
cegec said:
yes they are 6 arcseconds apart

Well what are you waiting for? You know the approximate proportions, and you know an angle. Make a schematic picture of what you know. The first step is always to get some rough solution---you can make it more accurate later.
BTW what are the two redshifts?
 

1. What is the Hubble constant?

The Hubble constant is a measurement of the rate at which the universe is expanding. It is denoted by the symbol H0 and has units of kilometers per second per megaparsec. It is named after astronomer Edwin Hubble who first discovered the expansion of the universe.

2. What is a gravitational lens?

A gravitational lens is a phenomenon where the gravitational force of a massive object, such as a galaxy or a cluster of galaxies, bends the path of light from a more distant object behind it. This creates a distorted or magnified image of the distant object, allowing us to study it in more detail.

3. How is the Hubble constant determined from time delay in a gravitational lens?

The time delay in a gravitational lens refers to the difference in arrival times of light from a background source that is lensed by a foreground object. By measuring this time delay, and knowing the distance to the lensing object and the background source, scientists can calculate the Hubble constant using a mathematical formula.

4. Why is it important to determine the Hubble constant accurately?

Knowing the Hubble constant is crucial for understanding the age, size, and expansion rate of the universe. It also helps us to better understand the nature of dark energy and the ultimate fate of the universe.

5. What other methods are used to measure the Hubble constant?

In addition to the time delay in a gravitational lens, scientists also use other methods such as the cosmic microwave background radiation, supernovae, and the Tully-Fisher relation to determine the Hubble constant. Each method has its own strengths and limitations, and combining multiple methods helps to improve the accuracy of the measurement.

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