# When measuring the speed at which far-flung galaxies move, do scientists factor in account that they are seeing the way the galaxies moved in the past? Could this impact Hubble's Law?

David Rothstein, a postdoctoral fellow in Cornell University's astronomy department, searches the universe for an answer to this question.

Our universe is expanding—astronomers have piled up observations, over many decades, which suggest that other galaxies appear to be moving away from our own Milky Way galaxy (and from each other) at fantastic speeds. There are some small deviations from this pattern, but if you were to "pan the camera back" and take in the universe as a whole, the overall sense would be that galaxies are rushing away from each other, with farther galaxies moving away proportionally faster—a paradigm known as Hubble's Law.

What would the universe look like from this point of view? A good analogy for the expanding universe comes from Martin Gardner, a popular science writer who was also a longtime columnist for Scientific American: Imagine a gigantic blob of dough with a bunch of raisins embedded throughout; the dough represents space, and the raisins represent the galaxies. Now, if someone puts the dough in the oven, it will expand or, more accurately, stretch, keeping the same proportions as it had before, but with all the distances between raisins getting bigger as time goes on.

Astronomers use something called the "Hubble constant" to measure how fast this expansion is taking place. The measured value of the Hubble constant can be written in many ways, but the way I like to write it is 0.007 percent per million years. This means that every million years, the distances between galaxies all stretch by around 0.007 percent.

So what does this number really tell us? For one thing, it tells us that the universe is very old. If one were to go back millions of years, the universe would look pretty much the same as it does now. As long as you stick to measuring galaxies within, say, a hundred million light-years of our own, you can be assured that the universe will not have changed much in the time it took light to travel from those galaxies to us.

But what if you're measuring a galaxy that's a few billion light-years away? In that case, the universe has changed significantly as the light has traveled. Astronomers no longer measure Hubble's Law for these galaxies due to a whole host of problems: If you were to try to measure the "distance" to one of these galaxies, which distance would you get? The distance when the light was emitted? Or the distance the light traveled to reach us (which includes some extra distance because the universe expanded while the light was moving through it, like a runner on an ever stretching racetrack)? Or would you measure the distance that the galaxy is from us currently, which is the largest of them all? Similar problems exist with speed: the Hubble constant changes with time, and depending on how it changes, individual galaxies might be speeding up or slowing down. So when you talk about speed, are you talking about the speed when the light was emitted, the speed now, or something in between? In short, it's all a big mess.

The way to get around this is to stop thinking about distance and speed and to focus on properties that astronomers can measure directly. One thing that astronomers can actually measure is the redshift—as light travels through the expanding universe, the light gets stretched by the same factor that the universe does, causing its wavelength to increase. Since red light has longer wavelengths than blue light, this means that the color of light will move more toward the red end of the spectrum. And instead of distance, astronomers look at objects of known power inside the galaxies (typically type 1a supernovae) and measure how bright they appear. This is a bit like taking a 60-watt lightbulb and moving it to farther and farther distances. As long as we can be sure that the bulb remains at 60 watts, we know that the fainter it appears, the farther away it must be.

View
1. 1. Neil_Polymathes 04:01 PM 1/5/08

I have a different conceptual principle to offer. One that contradicts the very idea of there being a Hubble constant. It explains why the measure changes with different sample groups and why when later revisiting sample groups the measure modestly changes. It also makes a modification to the principle factors governing Hubble law.

2. 2. hdchiba 09:02 PM 1/8/08

Why can we not see beyond those last galaxies? Is light speed too slow or is expansion to great? hdc

3. 3. mkirsh 03:11 PM 1/9/08

Dear Sirs:In the search for the stars mathematical constants bother me as much as constant or dogmatic and rigid thoughts or ways of thinking or the stretching of the imagination with mathematical possibilities and probalities.. We all each have our peculiar ways of thinking that are never exactly the same between any two humans. Whether distant, far, invisible, a universal truth does not have a number attached to it..the distance to a point from the back of ones head is not the same as the distance from the front of the head where the eyes can see. No living thing can see, and hence consider for his immediate living tasks, a whole 360 degrees. To do so could have no more a meaning than to assign a fixed age, or size to all humans. The lending of constant mathematical values in physics in correspondence assigns to space a description other than what ones knows-and has no meaning, less to also add in definition that incoherencies without a knowable definition can exist and also co- exist with normal two dimensional space that is perceived. This might be acceptable as experiment of the intellect, but is not suited as a tool to create heating furnaces, or stock kitchen cupboards, or nuture public perspective/the young, anymore than one would elect one single politician to sire all the newborn on the earth. Getting down to earth in such a case one not know one person from the other, and less than the actual projected remains of the low and lecherous battle for the earths resources. Mutual anonominity, in large scale group activities is universally a prerequisite for a victimization of the foreign. The teaching of the existence of such physcial constants, especially in the education of the young, in the absence of an understanding of the actual meaning as no more than pen and paper experiment; the pursuit of anonymous physical constants has no exact corresponding difference in meaning/in psycho lingual meaning, consequential action, than as a victimization of our own living spaces. Some politicians might not balk to be the stant fathers of tomorrow, but with an added enlightenment, might object to be the con-stant fathers of all tomorrow. http://www.marvinekirsh.com http://www.authorsden.com/marvinelikirsh

4. 4. woodjc 04:39 PM 1/9/08

Every good answer raises new questions. The energy of a photon is given by it's frequency multiplied by Planck's constant. If the light is being "red shifted" the energy in each photon must be reduced. What happens to this energy?

5. 5. Neil_Polymathes 06:14 PM 1/9/08

While I might be wrong, the answer is in the blog I put up today.
It explained the nature of red-shift without spacial Expansion theory.

The energy is lost to to spacial gravities.
Please let me know if the blogged idea is in agreement with you.

6. 6. jasonkidd007 03:38 AM 1/10/08

w

7. 7. jasonkidd007 03:39 AM 1/10/08

Which type would be least harmful to an organism why?
a) substitution versus inversion?
b) deletion vs. substitution?

8. 8. Darrylsb 11:43 PM 1/10/08

What if the shape of the universe was an expanding sphere similar to a planetary nebula created from the big bang? All of the galaxies we see are contained along the thick surface of the sphere. As the sphere grows the surface expands exponentially making it appear to be accelerating.

9. 9. Giulio 07:02 AM 1/31/08

What if the light from type Ia supernovae are being obsured to different degrees by dust and other space debris? What if what we think we observe is actually a mirage like the shimmer of hot asphalt in the distance?What if the laws of physics have changed over the course of time. A very simple example: I cannot swim through solid ice but I can swim through liquid water. The substance is the same, but it has changed due to time and temperature. The medium through which the light travels must remain the same over billions of light years. And we need to be sure that what we observe is reality and not illusion. There are so many variables to take into account that any theory or conclusion whether through observation or modeling is as good as anyone elses guess.

10. 10. P.Moore 09:25 AM 3/3/08

Most distant visible quasars are said to have an age of less than 1Gy compared to age of Universe of 14Gy. So how is it that if these quasars emitted light when they were at a distance of less than 1GLY it has taken this light about 13Gy to reach us?

11. 11. Primordial 10:43 PM 7/9/08

Because of the expanding universe, the Super clusters are moving away from other Super clusters, and Super clusters are composed of galaxies with super massive black holes at their cores, my question is about the mass of the super massive black holes at the cores, does their relativistic mass increase relative to each other or remain the same, because they are regressing relative to each other?

12. 12. WONDER BOY 11:29 PM 7/23/08

I DON'T THINK HE ANSWERED THE QUESTION YOU WERE LOOKING FOR.ASK THE QUESTION THIS WAY.
THE LIGHT FROM THE ANDROMEDA GALAXY IS 200 MILLION LIGHT YEARS OLD ,WHEN WE LOOK AT THAT GALAXY. THAT GALAXY IS MOVING DIRECTLY TOWARDS ARE GALAXY. SO ANDROMEDA HAS HAD 200 MILLION YEARS TO MOVE CLOSER TO US FROM WHAT WE ARE OBSERVEING. FOR ALL WE KNOW ANDROMEDA COULD BE CRASHING INTO ARE GALAXY RIGHT NOW, AND WE COULD NOT SEE IT. IS THIS OBSERVATION TRUE OR NOT?

13. 13. WONDER BOY 11:39 PM 7/23/08

IF THE ANDROMEDA GALAXY IS SAID TO BE 200 MILLION LIGHT YEARS FROM US AS WE OBSERVE IT, AND THE GALAXY IS MOVING TOWARDS ARE GALAXY. DOES THAT MEAN THAT ANDROMEDA HAS HAD 200 MILLION YEARS TO MOVE CLOSER TO US AND WE CAN'T SEE IT BECAUSE WERE SEEING THE LIGHT FROM IT , FROM 200 MILLION YEARS AGO. FOR ALL WE KNOW ANDROMEDA COULD BE CRASHING INTO ARE GALAXY RIGHT NOW?

14. 14. alphachapmtl 11:02 PM 8/6/08

"Imagine a gigantic blob of dough with a bunch of raisins embedded throughout; the dough represents space, and the raisins represent the galaxies. Now, if someone puts the dough in the oven, it will expand or, more accurately, stretch, keeping the same proportions as it had before, but with all the distances between raisins getting bigger as time goes on.
"
Are the raisins expanding also?
Space expands between galaxies.
What within galaxies? Between planets? Between electron and proton in hydrogen atom? I guess the answer is no, but is this consistent? How can space expand on large scale but not on small scale?

15. 15. alphachapmtl in reply to WONDER BOY 11:16 PM 8/6/08

The Andromeda galaxy is estimated to be at 772 +/- 44 kpc

1 kpc = 1 kiloParsec = 3262 ly (light-year)
772 kpc = 2518264 ly = 2.5 Million ly

" ... the Andromeda Galaxy and the Milky Way are approaching one another at a speed of 100 to 140 kilometers per second (62–87 miles/sec.).[18] The impact is predicted to occur in about 2.5 billion years."

http://en.wikipedia.org/wiki/Andromeda_galaxy#cite_note-Ribas2005-3
http://www.journals.uchicago.edu/doi/abs/10.1086/499161

16. 16. PeterROwen in reply to alphachapmtl 07:17 PM 3/15/10

Now, surely it will happen before that since as Amdromeda gets closer it gets "sooner".

You must sign in or register as a ScientificAmerican.com member to submit a comment.
Click one of the buttons below to register using an existing Social Account.

## More from Scientific American

• Climatewire | 27 minutes ago | 1

### Thawing Tundra May Produce Less CO2

• MIND Guest Blog | 1 hour ago

### What's Individuality, and Where Does It Come From?

• Cross-Check | 1 hour ago

### Why You Should Care about Pentagon Funding of Obama's BRAIN Initiative

• Beautiful Minds | 1 hour ago

### The Complexity of Greatness: Beyond Talent or Practice

• Extinction Countdown | 1 hour ago

More »

## Latest from SA Blog Network

• ### U.S. Hurricane Forecasts Could Be Better

Observations | 51 minutes ago
• ### Pirates, Charles Darwin, and One Very Un-Extinct Dodo

The Artful Amoeba | 57 minutes ago
• ### What's Individuality, and Where Does It Come From?

MIND
MIND Guest Blog | 1 hour ago
• ### Dividing Arcella (test construction in progress)

The Ocelloid | 1 hour ago
• ### Why You Should Care about Pentagon Funding of Obama's BRAIN Initiative

Cross-Check | 1 hour ago

## Science Jobs of the Week

When measuring the speed at which far-flung galaxies move, do scientists factor in account that they are seeing the way the galaxies moved in the past? Could this impact Hubble's Law?

X

### Subscribe Today

Save 66% off the cover price and get a free gift!

X

X

###### Welcome, . Do you have an existing ScientificAmerican.com account?

No, I would like to create a new account with my profile information.

X

Are you sure?

X