Is it a new theory?

[Florian]

Considering scientific standards, this is a fairly recent theory.
While it is quite true that the hypothesis has a long history, it consisted mainly of individuals arriving at the idea independently from the end of the XIXth century to the middle of the XXth century. The most prominent individuals of this period are the russian physicist Ivan Osipovich Yarkovsky (1888), the italian geologist Roberto Mantovani (1889 and 1909), the russian geologist M. Bogolepov (1930) and the german geophysicists B. Lindemann (1927) and Ott Christoph Hilgenberg (1933).
But the scientific development of the theory did not really begin until the 1950s with L. Egyed and Samuel Waren Carey:

"With Egyed the expansion hypothesis entered the normal literature of science...; and then the literature on expansion began an exponential growth."
William Menard, The Ocean of Truth, 1986, p. 145

"This idea [ the possibility that the earth's interior is expanding] may sound unfamiliar, but there is ample evidence in its favor, mainly collected since the activities of the Geophysical Year began in 1957."
Arthur Holmes, Principles of Physical Geology, 1965, p. 34

"[Carey's] results were first published in 1955 in Geological Magazine (vol.92, page 196) and full details in The Tectonic Approach to Continental Drift (Continental Drift, A Symposium, Geology Department, University of Tasmania, 1958)… A significant section in his long book within-a-book contribution to the proceedings (p.177-355) was devoted to the expansion hypothesis (p.311-349)."
Giancarlo Scalera,"Samuel Warren Carey: commemorative memoir" From Scalera, G. and Jacob, K.-H. (eds.), 2003: Why expanding Earth? – A book in honour of O.C. Hilgenberg. INGV, Rome, 85-95.