# Expanding universe?

1. Jun 4, 2014

### kenny1999

Universe is expanding which means the earth is getting away from the sun. Without the sun, it is probably that all organisms on the earth would die. Is there any calculation or estimation from scientists that how many years before the earth is too far away from the sun so that we will all die

2. Jun 4, 2014

### mattt

No. It does not mean that. It means that huge portions of about 300 million light-years (more or less) are getting away from each other.

In smaller scales (small group of galaxies, inside a galaxy, in our solar system) things are not getting away from each other (on average).

You'd better worry about the Sun turning to a red giant in about 7.10^9 years and destroying the Earth.

3. Jun 4, 2014

### Staff: Mentor

Partially true. The universe is indeed expanding. However, the expansion of space does not take place within galaxies and solar systems because gravity is strong enough at these distances to hold everything together. It is only when distances between objects gets REALLY big and gravity gets very weak that expansion starts to take over. So, in effect, expansion causes galaxy clusters to recede from each other, but nothing within a galaxy clusters recedes from anything else within the same cluster.

4. Jun 4, 2014

### Chronos

Just for fun, let's try doing the math. There are 9.46E15 meters in a light year and 3.26 million light years in a megaparsec. That makes for a total of about 3E22 meters per megaparsec. The expansion rate of the universe is about 70,000 m/sec/mpc which is roughly 1 m/sec for every 4.4E17 meters. Earth's distance from the sun is about 150 billion meters. That means [ignoring gravity] the earth and sun would currently be receding at a rate of 3.4E-07 meters per second, or about 10.7 meters per year. This same reasoning yields a recession value of the moon from earth [d=384 million meters] of 8.73E-10 meters per second, or about 2.75 centimeters a year. We have measured the actual recession speed of the moon [3.8 cm] and it is explained by tidal friction. The Lunar Laser Ranging experiment should have detected an anomalous extra 2.75 cm of recession per year if expansion played a role.

5. Jun 5, 2014

### kenny1999

The world should have already used up all the energy and resources and killed itself well before the sun destroying the Earth. Do you agree

6. Jun 5, 2014

### kenny1999

Why big stuff would have weak gravity?

7. Jun 5, 2014

### Bandersnatch

Well, what does r stand for? What happens to F when r is large?

8. Jun 5, 2014

### Staff: Mentor

They don't. But the distances between galaxy clusters is staggeringly large. Tens of millions of lightyears on average. This greatly reduces the gravitational attraction between.

9. Jun 5, 2014

### AgentSmith

But in the far future, the accelerating expansion will overcome gravity, tear apart super clusters, then clusters, galaxies, solar systems, the whole shebang. Only photons will be left wondering about in an empty universe.
Is this not the interpretation by most, well, many, cosmologists?

10. Jun 5, 2014

### Mordred

the big rip is invalidated, in order for the big rip to work the cosmological constant would have to gain energy density strong enough to overcome the strong force and gravity. However the cosmological constant has been shown to have a constant energy density. Instead we are heading for what is known as heat death.
http://en.wikipedia.org/wiki/Heat_death_of_the_universe
even then this fate is hypothetical

11. Jun 5, 2014

### bapowell

This is not right. We don't know that the dark energy driving the present-day accelerated expansion is actually constant. This is based on observational constraints on the equation of state parameter, w. While there are no statistically significant deviations from w = -1 (a cosmological constant), it is still perfectly likely that the equation of state is consistent with a varying energy density, even one leading to a big rip: http://arxiv.org/abs/1303.4353

12. Jun 5, 2014

### Mordred

hrmm interesting hadn't seen that paper before thanks, has there been any further findings in regards to a varying cosmological constant in the last year that your aware of?

Last edited: Jun 5, 2014
13. Jun 5, 2014

### enorbet

If I understand correctly some researches think we are in a "dark energy" phase of evolution of the Universe, considering that periods have existed in which relative importance, even existence, of determining factors have changed during succeeding epochs. Deductive reasoning often falls into the "if this, then that" category and that works but is not without limitations. If humans, like dragonflies, had lifespans of one day, it might be very difficult to imagine snow, let alone Winter, if you were born, say, during June.

That said, unless some major change occurs, and there is certainly enough time (and possibly, space) for such to occur, it is impossible or at least fruitless to speculate. "Smart Money" is on No Big Rip.

14. Jun 5, 2014

### CraigDxHypo

I think Cooperstock, Faraoni, Vollick’s 1998 The influence of the cosmological expansion on local systems contains a good technical paper explaining why the metric expansion of space isn’t noticeable on solar system scale. In section 4-"Cosmological corrections to the two-body problem in the LIF", they calculate a fractional change in period of orbit of 2.8e-33/year, which (courtesy of Kepler’s 3rd law) give an increase in the radius (more correctly, semimajor axis) moon’s orbit of 2.8e-33^(2/3)*386e6 = 7.6e-14 m/y, on the order of 1/10^22th the observed rate and the rate calculated by post #4’s straightforward application of the current Hubble’s constant

I don’t feel technical explanation are very intuitively satisfying, though. While it’s accurate to state that the reason Expansion is so much smaller on interplanetary scales than given by the Hubble constant, such statements don’t feel detailed enough. So I find it intuitively appealing to take a simple 2-body, one big, one tiny, in a circular orbit, simulation and add a small constant outward radial acceleration to the tiny body. Coincidentally, I recently ran a simulation for a solar sail at various angles relative to the direction of the Sun, which in the 90deg case and a very low acceleration, is nearly identical. What happen in this case is that the body doesn’t transfer to a different orbit, but follows a forced, precessing elliptical orbit with periapsis matching the initial circular orbit.

While Expansion causes no detectible increase in radius or orbit, perhaps it causes a very small deviation in the General Relativity-predicted precession of the orbit that could be detected? Alas, my GR physics skills aren’t up to calculating it.