"Say the universe's expansion is exponential decreasing, that would make the earth/universe much younger than scientists calculated."
You need to study what Nature is showing us.
http://www.astro.ucla.edu/~wright/cosmol...
... an excellent site, that should get you up to speed.
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Have one guy hold a tape measure on the other end of a room without moving. Wait a century or two. You will find that he moved away from you slightly.
After that, be sure to buy your friend a beer. He earned it.
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By observing two kinds Cepheid variables in the Milky way AND other galaxies over decades, over more than 80 years.
"...Hubble was lucky enough to arrive at Mt. Wilson soon after the 100-inch reflecting telescope was completed. A careful and hard-working observer, Hubble took many photographs of the same set of spiral nebulae (now called galaxies). Multiple images were needed in order to identify changes over time. He observed several novas, or instances in which a dim star became much brighter as it attracted material from a nearby companion star. Then, on October 4, 1923, while comparing a photograph that he had just taken of the Andromeda galaxy with photos taken on previous nights, Hubble identified a Cepheid variable star—the one kind of star that could provide a means of determining the distance to the galaxy. Over the next several months Hubble determined that the star varied in brightness with a period of 31.45 days, which meant it was 7,000 times brighter than the Sun. Comparing its apparent brightness with its actual brightness Hubble determined that it was 900,000 light years away.
Since Harlow Shapley had previously measured the distance across the Milky Way to be about 100,000 light years, the new findings clearly indicated that the Andromeda galaxy was far beyond the Milky Way. Later investigators found that there were two types of Cepheid variable stars, and that Hubble was comparing the bright kind of Cepheid in Andromeda with a dimmer kind of Cepheid in our own galaxy, which meant that Andromeda was actually twice as far away—approximately 2 million light years. In subsequent decades, distances were measured to many other galaxies. Today, galaxies that are billions of light years distant have been observed.