# Define hubbles law expresses a relationship between

### Hubble's Law | Astronomy Planets, Stars, Galaxies, and the Universe

Edwin Hubble's plot of the Velocity-Distance relationship for galaxies, shows a linear What this means is that as the distance gets bigger, so does the velocity. A rising loaf of raisin bread is a good visual model: each raisin will see all other Hubble's law is a statement of a direct correlation between the distance to a galaxy The red shift of the spectral lines is commonly expressed in terms of the . Hubble discovered a relationship between two measurable properties of If all objects are moving outward at a constant speed, the boundaries defined by the.

The first is this: The dots are supposed to represent galaxies. If you pull on the rubber band, the distance between the dots will grow.

Click on the expand button in the window below to see this demonstrated. If the initial distance between each dot is 1 cm Dot B is 1 cm away from Dot A, Dot C is 2 cm away, and Dot D is 3 cm away and you pull on the rubber band so that the dots are now 2 cm apart, then from Dot A, Dot B will be 2 cm away, Dot C will be 4 cm away, and Dot D will be 6 cm away.

Therefore, from Dot A's point of view, the more distant dots will appear to have moved faster than the closer dots remember, the velocity of an object is the distance traveled divided by the time it takes to go that distance. If we were to repeat this experiment, but measure the distances between the dots from Dot B's point of view, we would find that Dot B would draw the same conclusion as Dot A. That is, all the dots would appear to be moving away from Dot B, and the farther dots would appear to move faster.

### ESA Science & Technology: Galaxies and the Expanding Universe

The galaxies are not really moving through space away from each other. Instead, what is happening is the space between them is expanding just like the rubber band expanded, separating the dots fixed to it from each other. As the universe expands, the galaxies get farther from each other, and the apparent velocity will appear to be larger for the more distant galaxies.

The Earth and the Milky Way are not special in seeing that all galaxies appear to be moving away from us. If we were on a different galaxy, we would also see all the other galaxies appear to be moving away from us because of this expansion.

Instead of a line, picture the dough for raisin bread. Inside the dough, all of the raisins are separated from each other. As the dough rises during baking, all of the raisins will move farther away from each other. Let's say that the size of the dough doubles. The distance between all of the raisins will double, and just like the dots on the rubber band, the more distant raisins will appear to have moved faster.

## Hubble's constant

Animated image showing the rising of a loaf of raisin bread dough. This animation contains the same idea as in Figure The raisins in the dough are like the dots on the rubber band or galaxies in the Universe. As the dough expands, the separation between the raisins increases, just like the separation between galaxies in our Universe.

We use one more analogy to try to explain the mathematics of the expansion of the universe and to answer another common question that arises in cosmology: Why can't we observe the center of the expansion?

Picture a universe that consists only of the surface of a balloon. All of the galaxies and the stars in the galaxies are fixed onto the surface of the balloon. There is no way for the observers to perceive the region inside the balloon or the region outside the balloon, they are and light is constrained to travel only along the surface. In this analogy, as the balloon inflates, the galaxies on the surface of the balloon will move farther away from each other. Just like with the rubber band and raisin analogies, if you measure the distance between the galaxies before and after the inflation of the balloon, you will be able to show that the more distant galaxies will appear to move faster, just like Hubble's Law in our universe and like the rubber band and raisin loaf experiments.

Again, every galaxy will observe the same effect, and no one galaxy is in a special location. If you ask where the center of the expansion is, it is inside the balloon. This means that no location on the surface of the balloon the universe according to the residents on the surface of the balloon can be identified as the "center" of the universe.

We use this analogy to answer the question: Where is the center of our universe? The idea is that we live in a universe with three spatial dimensions that we can perceive, but that there exist "extra" dimensions maybe one, maybe more than one that contain the center of the expansion.

Just like the two-dimensional beings that inhabit the surface of the balloon universe, we cannot observe the center of our universe. We can tell that it is expanding, but we cannot identify a location in our 3D space that is the center of the expansion.

Until this point, we have been describing the redshift of light as a Doppler shift. However, now that we understand the Universe to be expanding, we need to revise this description.

For example, the galaxy M31 does not even show a redshift; it is blueshifted, showing that its peculiar velocity is pointed towards us, rather than away from us. Recall the concept of the "lookback time" for an object. For objects at very large distances from us, it is very common to see their distances referred to not in units like parsecs or light years, but in units of time.

You can consider Hubble's Law to be the final rung in the distance ladder. If you know Hubble's constant accurately, then you can calculate the distance to any galaxy in the Universe simply by measuring its velocity which is reasonably easy to do for any galaxy for which you can observe its spectrum. To calibrate Hubble's constant, though, you need to be able to plot the distances for a number of galaxies as obtained using other methods. While that may seem like an easy statement to make, it was an incredibly difficult task to accomplish.

For decades, astronomers have argued over the precise value of Hubble's constant. This measurement was, in fact, one of the major reasons for building and launching the Hubble Space Telescope.

It spent years observing Cepheid variables in distant galaxies in order to measure Hubble's constant as precisely as possible. The results were reported in