阅读短文，回答问题：

星系在远离我们而去

当一列火车迎面驶来时，我们听到的汽笛声音调会升高（即声波的频率升高）；当它离我们远去时，音调降低（即声波的频率降低），这一现象叫多普勒效应．

光同样可以产生多普勒效应，当一个光源靠近我们或离我们远去时，我们观测到它发出的光的频率也会发生变化，变化的规律也会发生变化，变化的规律与声音发生多普勒效应的规律相同．

20世纪天文学家哈勃观察到，几乎所有远处星系发出光的频率都向红光方向偏移，称为“红移”，这一现象说明，星系在远离我们而去，哈勃研究了24个距离已知的星系，由它们进行的速度v与它们离我们的距离r的图象，如图所示，并据此，得出它们退行的速度v与它们离我们的距离r的关系﹣“哈勃定律”．

为解释哈勃定律，科学家提出“宇宙大爆炸”理论，认为宇宙是从一个大爆炸的火球开始形成的，假设我们就位于其中心，大爆炸后各星系以不同的速度向外匀速运动，远离我们而去．

阅读理解

    Remember Tony and Peter．the twin brothers who started singing at the age of five?The little boys are now twenty—five years old and will come back home for the first time since they left ten years ago．They will sing to celebrate the 100th birthday of our beautiful town．

    Tony and Peter are famous for their wonderful voices．They are popular with both young and old people．Three years ago,the brothers organized the Garbage Can with their three cousins．The five of them started to write their own songs and have become successful．In the coming concert in Greentown,the Garbage Can is going to sing their bestloved songs and several new ones．

    The concert will be at Apple Park at 7 pm on 10th october．Fans who do not want to miss it should get to the park as early as possible．