In 1913, astronomer Vesto Slipher—who, despite his exotic-sounding name, was from Mulberry, Indiana—was studying the light of spiraling nebulae (what were later discovered to be galaxies). He knew that, as celestial objects move away from us, their light gets stretched out, causing it to shift to a different section of the color spectrum– toward the red end. In this phenomenon, called redshift, the appearance of the light depends on the observer and the observed: a celestial object’s light appears redshifted to us, on Earth, because it is moving away from us.

But what wasn’t known—and what astronomers later found out—was that it was not only the movement of galaxies away from us that causes redshift. The universe’s effect on light waves as they thread their way through the cosmos is important as well. The path those galaxies’ light takes to reach us—perhaps as we’re standing in a quiet field in Indiana, the only interruption of light a few lamps from a small town—cannot avoid the pull of our expanding universe.

To picture light waves, think of water shooting out of a garden hose. The water that first comes out of the hose, depending on the force of the stream, lands several feet away from the hose’s mouth. For the moment, ignore the idea of gravity, so your line of water never hits the ground; it just persists as a continuous stream of hydrogen-oxygen molecules.

Imagine yourself moving the hose up and down, as if you’re shaking hands with someone (though don’t actually shake hands with someone while holding a hose spraying water). The result could be a little wet and less than congenial. But envision the trajectory of the stream as you raise and lower your hand. The water would arc in rhythmic crests and troughs—in rhythmic hills and dales. It might even sparkle in the sun, if you are wont to imagine a sunny summer’s day when you might actually be performing such maneuvers with your garden hose.

Speed up the raising and lowering of your hand—as if you’re very excited to meet the person you’re shaking hands with—but do so at the same height and base as before. The stream of water’s crest (the top of its hill) and its trough (the vee of its dale) are much closer together now. You’ve increased the frequency of your water’s wave.

This wave is traveling like a wave of light travels. And the frequency of light—how close together its crests and troughs are—determines what color we see in the light. If something looks blue—your blue notebook lying next to your feet, waiting for your observations—it is because it reflects light at you within a high frequency range. At the opposite end of the visible rainbow is red. If your notebook is red, perhaps the color of a ripening mulberry, it reflects light to you at a low frequency.

Now, think again of those redshifted galaxies. Those galaxies do not move toward the red end of the light spectrum simply because they are moving away from us. Those galaxies redshift because, as Edwin Hubble discovered in 1929, the universe is expanding. As the universe balloons outward, it stretches space-time and likewise elongates the galaxies’ light waves toward the red, an effect aptly named cosmological redshift.

With your hose, we are now going to defy the laws of gravity and properties of water. Imagine your stream of water as solid—not frozen water, but water you can stretch. That friend you were going to shake hands with? Ask her to grasp the end of the water stream. Turn off the hose. Direct her to walk away from you. As she walks, observe how the crests and troughs of your stream lengthen. As it lengthens, think of a wave of light and how its color changes with the size of its wave. Take note of your hose’s, your wave’s, gradation from blue to green to yellow—extending ever longer—to orange, and finally red. Write this down in your red or blue notebook. You are the observer; she is the universe.

This is what happens with the light waves reaching us from distant galaxies. Because the universe is expanding, it broadens those waves, stretching them as they move up and down. Though the galaxies might not appear red to the observer, their light’s frequency is moving that way. And if the waves are drawn out enough, if the galaxies are flung far enough, they may enter the range of visible light that to us—in that chirping, dark meadow in Slipher’s Mulberry, Indiana, at the end of that summer’s day during which we were playing with hoses, running through sprinklers, washing off the dust of the dirt road outside our house—appears red.