Kicauan Jagat Raya Bukti Gelombang Gravitasi, Apa dan Bagaimana Menemukannya?
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بِسْــــــــــــــــمِ اﷲِالرَّحْمَنِ اارَّحِيم
الحمد لله رب العالمين, وصلاة والسلام على أشرف المرسلين. أما بعد :
الحمد لله رب العالمين, وصلاة والسلام على أشرف المرسلين. أما بعد :
Penemuan gelombang gravitasi membuat dunia ilmiah gegap gempita, tetapi mungkin membuat publik mengernyitkan dahi. Sementara kalangan ilmuwan bisa menyatakan bahwa penemuan ini membuktikan kebenaran teori Albert Einstein dan membuka era fisika baru, dunia awam mungkin masih bertanya-tanya, apa sebenarnya gelombang gravitasi dan bagaimana penemuannya.
Apa itu gelombang gravitasi? Gelombang gravitasi adalah sebuah gangguan atau riak di alam semesta yang mahaluas, bisa diibaratkan sebuah riak di kolam tenang yang terbentuk ketika kita mencelupkan dan menarik jari tangan kita di dalamnya.
Namun demikian, tak seperti riak di kolam yang terlihat jelas, riak kosmos ini begitu misterius sehingga tak seorang pun bisa melihat, mendengar, atau merasakannya dengan indera keenam sekalipun.
Meski disebut kicauan alam semesta, gelombang gravitasi juga bukan gelombang suara yang memerlukan medium untuk merambat. Gelombang itu bisa merambat dari jarak miliaran tahun cahaya dan sampai ke bumi tanpa perantara!
Gagasan adanya gelombang gravitasi tak lepas dari Teori Relativitas Umum Einstein yang dikemukakan pada tahun 1916. Dalam teorinya, Einstein menyatakan bahwa alam semesta adalah kain empat dimensi.
Gelombang gravitasi dihasilkan oleh obyek apa pun di alam semesta yang mengalami perubahan kecepatan ataupun arah. Besar gelombang bervariasi tergantung obyeknya.
Bumi sendiri bergerak mengelilingi matahari dan kecepatan serta arahnya pun bervariasi walaupun relatif konstan. Jadi, bumi juga menghasilkan gelombang gravitasi.
Dalam konteks penemuan terbaru kali ini, gelombang gravitasi dihasilkan oleh dua lubang hitam yang masing-masing berukuran 36 dan 29 kali massa matahari.
Dua lubang hitam itu telah "berpacaran" selama miliaran tahun. Mereka semakin mendekat dari masa ke masa. Artinya, kecepatan berputar satu sama lain pun terus berubah sehingga menghasilkan gelombang gravitasi.
Akhirnya beberapa waktu lalu dua lubang hitam itu kawin. Mereka bersatu menjadi lubang hitam yang luar biasa massif, berukuran 62 kali massa matahari.
Persatuan itu menghasilkan gelombang gravitasi yang luar biasa besar. Besarnya bisa disetarakan dengan selisih antara jumlah massa lubang hitam yang sebenarnya dengan massa lubang hitam baru yang terbentuk.
Dua lubang hitam bermassa 36 dan 29 kali matahari seharusnya membentuk lubang hitam bermassa 65 kali matahari. Namun, yang terbentuk ternyata 62. Sisa 3 kali massa matahari itu yang dikonversi menjadi energi gelombang gravitasi.
Ada beberapa usaha untuk mendeteksi gelombang gravitasi. Temuan baru kali ini datang dari observasi dengan fasilitas Laser Interferometer Gravitational-wave Observatory (LIGO).
LIGO mendeteksi gelombang gravitasi berdasarkan perubahan yang diakibatkannya pada ruang dan waktu sekitarnya. Gelombang gravitasi akan meregangkan ruang dan waktu di satu sisi, tetapi memampatkan lainnya.
LIGO sebenarnya merupakan satu set fasilitas yang terpisah 4 kilometer, di Washington dan Lousiana. Keduanya dihubungkan oleh pipa vakum.
Dalam fasilitas LIGO, ada instrumen penembak laser. Laser akan terus bergerak dalam saluran vakum dari fasilitas satu ke yang lain.
Jika tak ada gelombang gravitasi, jarak yang ditempuh laser akan selalu sama sepanjang waktu. Namun, bila ada gelombang gravitasi yang datang, jarak tempuh akan berubah.
Perubahan itu yang ditangkap. IGO dilengkapi oleh detektor mahasensitif yang bisa menangkap perubahan yang sebenarnya amat kecil itu.
Bayangkan, perubahannya bukan lagi dalam ukuran milimeter, melainkan lebih kurang hanya sepersepuluh ribu dari diameter sebuah atom!
Detektor telah dirancang sesensitif mungkin dan seselektif mungkin sehingga perubahan karena adanya truk lewat dekat fasilitas, petir, atau lainnya yang bukan gelombang gravitasi diabaikan.
LIGO dikembangkan sejak tahun 2002. Tahun 2010, fasilitas LIGO dihentikan sementara untuk perbaikan. Pada September 2015 lalu, LIGO beroperasi kembali.
Fasilitas LIGO setelah perbaikan dikenal dengan Advanced LIGO. Singkatnya, fasilitas baru itu jauh lebih sensitif dan selektif dalam menyaring kebisingan.
Advanced LIGO sebenarnya baru beroperasi pada 18 September 2015 lalu. Namun, detektor yang telah dinyalakan beberapa hari sebelumnya ternyata berhasil mendapatkan data pada 14 September 2015.
Data itulah yang kemudian dikonfirmasi sebagai petunjuk gelombang gravitasi. Marco Drago dari Max Planck Institute for Gravitational Physics adalah orang pertama yang mengetahui adanya kicauan itu.
Drago menjumpai kicauan sebagai lonjakan frekuensi gelombang, dari normalnya 35 Hertz lalu memuncak menjadi 250 Hertz dan tiba-tiba saja turun lagi.
Selain LIGO, fasilitas lain yang berusaha melacak gelombang gravitasi adalah Laser Interferometer Space Antenna (LISA). Fasilitas ini dikelola oleh Badan Antariksa Eropa (ESA).
Ada juga North American Nanohertz Observatory for Gravitational Waves atau NANOGrav. Beda dengan LISA dan LIGO, NANOGrav tidak menggunakan laser, tetapi melacak gelombang radio.
